At a support-group meeting in June at Mount Sinai Hospital in Manhattan, Lori Silverman proudly announced that in the last seven months she had lost 106 pounds. Ms. Silverman, 42, who used to weigh 306 and hopes to reach 120, won a round of applause.

One by one, others reported similar victories: 140 pounds in four months, 85 pounds in seven months, 94 pounds in three months. And so on. More pounds lost, more clapping, more cheers -- testimonials from people who, thanks to steep weight losses, no longer needed medicine for diabetes, high blood pressure, high cholesterol, heartburn, gout or arthritis. Some had become able, for the first time in years, to fit into movie and airline seats, to reach down and tie their own shoes.

No diet got the credit. These people, who had all been massively overweight, had tried and failed at nearly every diet invented.

Their extraordinary transformations were brought about by surgery -- but not liposuction, which removes only a few pounds, or ''stomach stapling,'' which often fails.

Rather, these patients had major operations that simultaneously sealed off most of their stomachs to decrease the amount of food they could eat, and rearranged their small intestines to reduce the calories their bodies could absorb. The operations, which can be done in different ways, are known collectively as bariatric surgery, a term coined from the Greek words for ''weight'' and ''treatment.''

The surgery, which is becoming increasingly popular, mirrors the rise in obesity and the failure of diet, exercise and weight-loss drugs to help. In 1990, 44 percent of Americans were heavier than their ideal weight; now, more than half are. From 15 percent to 35 percent of adults are on diets, and Americans spend $30 billion to $50 billion a year trying to lose weight. An estimated 3 percent to 5 percent of the population of the United States, at least eight million people, more than half of them women, are morbidly obese.

Obesity is now one of the United States' worst public health problems. It greatly increases the risk of illnesses that are among the nation's leading causes of death, including diabetes, heart disease, respiratory problems and some forms of cancer.

Theories blame everything from genetics to sedentary lifestyles and the glut of fast food everywhere for rising weight. But regardless of the cause, experts on obesity have been forced to acknowledge that for the majority of obese patients, diet, exercise and weight-loss drugs simply do not work.

The surgery is not a quick fix or easy way out. It is a drastic step, carrying all the pain and risk of any major abdominal operation. It forces people to change their eating habits radically, makes them violently ill if they overeat and puts them at lifelong risk for major nutritional deficiencies.

But it works. Unlike any drug or diet, the surgery produces large, lasting weight loss in the majority of patients. Many lose 100 pounds. Some reach a normal weight, while others remain heavy, though less overweight than before.

The number of bariatric operations in the United States has doubled to 40,000 a year, from 20,000 just five years ago. About 80 percent of the patients are women. Most are young and middle-aged adults, but some surgeons accept patients in their 60's, and some even operate on teenagers. Entire families have had the surgery.

The number of surgeons performing it has risen sharply in recent years. The American Society for Bariatric Surgery reports that since 1995, its membership has increased by 16 percent a year, and now includes about 500 surgeons. Many major medical centers in the United States have hired bariatric surgeons, and many have waiting lists of patients. The surgery is on the rise in Europe, Asia and Latin America, as well; a recent conference attracted doctors from more than 20 countries.

''It's just exploding,'' said Dr. Latham Flanagan, a surgeon from Eugene, Ore., and a former head of the surgeons' group.

Dr. Elliot R. Goodman, who does bariatric surgery at Montefiore Medical Center in New York, said many hospitals were eager to tap into the market.

''It's a moneymaking venture for them if you can do it safely and get a length of stay of four to five days,'' Dr. Goodman said.

Bariatric surgery is a remedy of last resort, offered only when all else has failed, and only to the fattest of the fat, people who meet the definition of morbidly obese, which generally means overweight by 100 pounds or more. Some weigh over 600 pounds.

Bariatric operations cost about $20,000, and although many patients have had to fight for coverage, more insurers are beginning to pay, recognizing that the surgery can have powerful medical benefits that save them money in the long run.

Dr. Jules Hirsch, an obesity expert at Rockefeller University in Manhattan, called bariatric surgery ''a counsel of despair.''

''In a sense,'' Dr. Hirsch said, ''I think it's an expression of failure. It's an extraordinary thing to do to somebody to counter obesity, which has to come about by overeating and underactivity. But the only alternative is to stay the way you are or to adhere very strictly to a diet. It's extremely difficult for most people to do that. A lot of people, unfortunately, are popping up in our society that weigh 400 pounds.''

Even Dr. Hirsch and others who regard bariatric surgery as a gesture of defeat acknowledge that it works. In a 1991 report, the National Institutes of Health concluded that surgery was a reasonable solution for some people and said, ''a major drawback to the nonsurgical approach is failure to maintain reduced body weight in most patients.''

With drugs or diet, most patients can realistically hope to lose no more than 10 percent of their body weight -- a mere 30 pounds in someone who weighs 300, for instance -- and even that may not last, Dr. Flanagan said.

In contrast, studies show that 10 years after the most commonly performed bariatric operation, called gastric bypass, patients on average have maintained a loss of 60 percent of their excess weight. At East Carolina University School of Medicine, in Greenville, N.C., a research team led by Dr. Walter Pories has kept records on 608 patients and found that after 14 years, the percentages translated into an average weight loss of 100 pounds.

Physically and emotionally, the benefits are profound. Often, diabetes, high cholesterol and high blood pressure simply go away. So do breathing problems, including sleep apnea, a life-threatening disorder that forces many obese patients to sleep with oxygen masks. Ulcerated leg veins heal, as do crippling back and joint problems.

Patients no longer face the social stigma attached to being fat, or the scores of small, corrosive daily indignities, like being unable to squeeze through turnstiles or find clothes that fit. Their children are no longer ashamed to have them show up at school. Some who were on disability can go back to work. Women who had been infertile because of hormonal problems linked to obesity find that they can have children.

''Their lives turn around, open up,'' Dr. Flanagan said. ''They're no longer trapped in this prison of fat.''

Robert Altman, 56, who had bariatric surgery two years ago, said he owed his life to his surgeon, Dr. William Marcus, of Silver Spring, Md.

Mr. Altman, who is 5-foot-9, used to weigh 388 pounds. He could barely squeeze behind the steering wheel of his car or climb a flight of stairs, and could no longer work in his garden or pick up his grandchildren. He had diabetes, sleep apnea, gout, swollen legs, hypertension and high cholesterol. His doctor told him he might well be dead within five years.

Today, he is 220 pounds lighter, and his medical problems have cleared up. He needs vitamins and protein supplements to avoid malnutrition, but he does not mind. At 168 pounds, trim and smartly dressed, he bears no resemblance to his ''before'' picture.

''This is miraculous,'' he said. ''It's a new lease on life.''

High Benefits, High Risks

The benefits of surgery come at a price. Like any major operation, bariatric surgery carries serious risks, including the possibility of infection and complications from general anesthesia. The overall death rate from gastric bypass is one-half of 1 percent to 1 percent.

An estimated 10 percent to 20 percent of patients need follow-up surgery for complications. The risk of gallstones is increased after the surgery, and the National Institutes of Health estimates that nearly 30 percent of patients develop nutritional deficiencies that can result in anemia and osteoporosis. Vitamin and mineral supplements can prevent the deficiencies, but patients need to see doctors and nutritionists regularly for the rest of their lives. And unless they alter their eating habits drastically, which the surgery makes it easier to do, they can regain a lot of weight.

A death last year was a harsh reminder that the surgical risks are real. The patient was Christi Finch, 54, a founder of Word of Mouth, a highly successful gourmet shop in Manhattan.

When Ms. Finch entered St. Luke's-Roosevelt Hospital in Manhattan in August 1999 to have a gastric bypass, she weighed more than 400 pounds, had difficulty breathing and needed canes to walk.

Serious complications set in after the surgery, and Ms. Finch had to be rushed back to the operating room several times.

She never recovered.

In March, after seven months in the intensive care unit, she died from sepsis, a systemic infection caused by extensive bedsores.

Given Ms. Finch's immobility and breathing problems before surgery, her sister, Sharon Borgford, questioned whether she had really been a suitable candidate for a major operation.

In an interview, Ms. Finch's surgeon, Dr. Louis Flancbaum, confirmed the family's account of her illness and death but said the heaviest patients with the worst problems were the ones who needed bariatric surgery the most, because they had the greatest risk of further illness and death from being so obese. He said he almost never turned a patient away, and had even operated on people weighing more than 700 pounds.

Other surgeons agreed that the highest-risk patients were among those who needed the surgery most. Dr. Flanagan said that of the 900 patients he had operated on, seven died, all of them at great extremes of morbid obesity.

A History of the Surgery

Bariatric surgery was born 40 years ago when doctors noticed that patients lost weight after stomach or intestinal operations for conditions like cancer and ulcers. The first operation that came into use for severe obesity worked by causing ''malabsorption.'' Known as the intestinal bypass, it rerouted food past most of the small intestine, preventing the body from absorbing too many calories.

The operation produced big weight losses but caused serious malnutrition and some deaths from liver failure. It was abandoned. Later operations left the intestines alone and depended on ''restriction'' to bring about weight loss. The procedure used a combination of staples and a band to transform the stomach into a little pouch that would hold little food and would cause nausea and vomiting if a person ate too much.

The restriction procedures often worked initially but tended to fail over time, with many patients regaining lost weight. They ate too much and the stomach stretched, or staples loosened or the bands slipped. Complications like frequent vomiting were also common, and many patients needed repeat operations to repair the first one or just reverse it.

Gradually, restriction operations fell out of favor. Some surgeons still offer them, but many feel that the procedures, familiar to the public as ''stomach stapling,'' gave the entire field a bad name.

Today another operation, which remodels both the stomach and small intestine, thus combining malabsorption and restriction, accounts for 80 percent of bariatric procedures. Like the older restriction technique, the newer one, called the Roux-en-Y gastric bypass, uses staples to close off most of the stomach, leaving just a small pouch that can hold only about two tablespoons of food. But today's stapling technique is less likely to fail than that used in the earlier restriction operations. And unlike the earlier procedures, the gastric bypass also alters the small intestine, bypassing its first segment, the duodenum, and part of the second segment, the jejunum. The bypass is less extreme than those of past intestinal operations, but still reduces absorption of food. The surgery can be reversed if needed, but that is rarely done.

Weight loss starts immediately and continues for 18 months to two years, when most people achieve their maximum loss, usually 70 percent to 80 percent of their excess weight. Many then regain a small amount and stabilize, so that the average loss over time winds up being 60 percent of excess weight. People who take up regular, vigorous exercise, like running or stationary bicycle spinning classes, often achieve greater long-term weight loss. Many patients who lose a lot of weight, especially older ones, find that they need plastic surgery to tighten up loose skin and remove abdominal folds that can hang to the knees.

Gastric bypass seems to work in large part by enforcing a strict low-calorie diet. At first, patients lose their appetites and cannot eat more than a few bites at a time without feeling sick. For a few months, most can take in only a few hundred calories a day.

If they eat too quickly, or eat sweets, most will suffer ''dumping syndrome,'' which occurs when too much food or sugar passes too quickly into the small intestine. Dumping causes nausea, weakness, sweating, faintness and sometimes diarrhea, and most patients say it is so awful that they will do anything to avoid it.

After a year or so, as the pouch stretches, most patients can consume 1,200 calories a day, but doctors urge them not to exceed that. It also becomes possible to cheat. People who nibble cookies or potato chips all day, or sip milkshakes, can ''out-eat the pouch'' and get fat again, doctors say. For that reason, surgeons insist that the operation is merely a tool to help patients lose weight and that it will not work if they misuse it.

For the very heaviest people, particularly those who eat compulsively, some surgeons recommend operations more extreme than the gastric bypass. In those procedures, the patients are left with a larger stomach pouch and a more extensive intestinal bypass. They can eat more, but absorb less. The operations cause more weight loss than gastric bypass but can also cause severe diarrhea and flatulence and pose a greater risk of nutritional deficiencies. Surgeons disagree about whether to use the more extreme operations.

The Emotions of Eating

Many patients say that the enforced restriction on eating is critical to the success of gastric bypass. The need to avoid feeling violently sick from the dumping syndrome is a powerful brake on eating; dumping did not occur with the old stomach stapling procedures, and it was easier to overeat. Some patients freely admit to former lives of tremendous overeating, consuming whole pies or pans of brownies or six quarter-pound hamburgers at a sitting.

What creates such a drive to eat is not fully known.

Asked why he ate so much, Mr. Altman laughed and said: ''Joy. Sadness. Loneliness. Boredom. Food was always my best friend.''

Ms. Silverman said, ''I guess I was just filling a void in myself, feeding my emotions as opposed to my stomach.''

Another patient said she ate because she was hungry all the time, and until she had surgery had never known what it meant to feel full.

Dr. Goodman said: ''I think there's a great psychological overlay. It may be the idea that there's some solace in food. We see a high degree of physical and sexual abuse in patients like this. One of our patients had a terrible history, a rape history, and our psychologist felt that by putting on weight she was somehow insulating herself from the interest of men.''

But Dr. Goodman said he doubted that emotional factors alone could bring on massive obesity in most people.

''I don't think there's a morbidly obese patient sort of lurking in every normal-size person,'' he said. ''I think there are a subset with a genetic predisposition.''

Some people may be genetically programmed to form more fat than others from the same amount of food, and some may truly feel hungry all the time, perhaps because of flaws in the signals between the brain and digestive tract that normally create a feeling of fullness and turn off the urge to eat.

Scientists say that long ago, the ability to get fat was probably an advantage. In earlier days of human evolution, people faced cycles of feast and famine, and survival may have hinged on the ability to store body fat for the lean times. But today, in developed countries where life has become an endless feast, a genetic makeup shaped by famine may lead to endless weight gain.

Dr. Philip Schauer, who does bariatric surgery at the University of Pittsburgh, said: ''Patients who come to us, all their lives they've been overweight. Typically they've been on 10 to 20 major diets. They lose 10 or 15 pounds here and there and gain it back, and more. They are constantly battling obesity, and it's an unfair fight, for reasons that are not clear. Surgery gives them a tool to make this battle a fair fight.''

By the time they decide to have surgery, most patients are desperate. Cheryl Grieco, 42, a nurse who had a gastric bypass at the University of Pittsburgh, said: ''I had fought with my weight all my life, since I was 5. I struggled and struggled. Every day of my life I'd get up and try to figure out how I could get thin.''

She was 5-foot-2 and weighed 258 pounds. At work, racing breathlessly to patients' rooms in emergencies, she wondered if she would wind up needing resuscitation herself.

Seeing a friend lose 120 pounds from the surgery led her to consider it, but she thought about it for a year before going through with it in April, with Dr. Schauer.

In the first three months, she lost 77 pounds and six dress sizes. ''It's going great,'' Ms. Grieco said. ''It's everything that diets weren't.''

She walks and bicycles now, and hopes to get her weight down to 115. ''To me it's been a lifesaver,'' she said.

Ms. Silverman, too, had given up hope of losing weight without surgery. ''I was 306 pounds, 5-foot-1 1/2,'' she said. ''I couldn't breathe. I would get up and it was such an effort. My back felt like a twig about to snap. I got to the point where no diet was working for me. That's when my doctor said: 'You have to do something. You're going to die.' ''

She was miserable emotionally, as well, after struggling with obesity all her life. She had been thin just once, for a couple of shining years beginning in 1983, when she managed to diet down to 126 pounds. She keeps a photograph of herself from that period tacked up in her cubicle at the law firm in Manhattan where she works as a paralegal. She was strikingly attractive then, which made it all the more painful for her to see her weight surge out of control.

''I had such a hatred for myself,'' she said. ''And you don't realize the rejection that goes with being heavy. People are prejudiced. I understand. But it really hurts. They hit you in the soul.''

She recalled long-ago neighbors who would pointedly tell her how pretty her thin sister was, and times when she took a seat on a train, only to have the person beside her spring up and move away. When she lost weight, people told her how wonderful she looked and how awfully fat she had been.

''Then you gain it back and there's silence,'' she said. ''You know what the creeps are thinking.''

Ms. Silverman had a gastric bypass at Mount Sinai in November 1999, performed by Dr. Michel Gagner, who does the procedure laparoscopically, operating through about a half-dozen small openings. Dr. Gagner does five such operations a week.

By this month, Ms. Silverman had lost 127 pounds. ''I look in the mirror and I swear I'm looking at a stranger,'' she said. ''I'm still shocked.''

But it has not been easy. ''Believe me, you walk around nauseous a lot,'' she said. ''I walk around queasy and feeling green a lot of the time, and I'm afraid of being sick while I'm commuting or at work.''

By trial and error, she has identified an odd array of foods that agree with her: mussels, certain fruits and vegetables, cottage cheese, guacamole.

Emotionally, the course has been rocky. Ms. Silverman said she must deal with her anger at people who seemed to shun her when she was heavy but now act friendly. She has also felt depressed.

''Where food used to be my crutch when I was aggravated, I can't turn to it anymore,'' she said. ''You lose not just your weight, but your security blanket.''

In the meantime, she has begun to look more and more like the pretty young woman in the photograph.

''For all my complaining, I have no regrets,'' she said. ''I was a lot worse before. The weight loss makes up for everything. I've never had life. I don't know what it's about. I've always been on the outside looking in. I believe I'll be given a chance at life that I never had.''

The Fat Epidemic

Later articles in this series will investigate the lives of chronic dieters, overweight children, new research on obesity and excercise and the diet business.

Photos: Robert Altman, with his wife, Ellen, in 1991, weighed 388 pounds before bariatric surgery two years ago. Now he weighs 168 pounds. (Marty Katz for The New York Times)(pg. A1); Lori Silverman, with her nephew in January and recently at her job at a Manhattan law firm, has lost 127 pounds since having bariatric surgery last November. ''I look in the mirror and I swear I'm looking at a stranger,'' she says. ''I'm still shocked.'' (Thomas Dallal for The New York Times); The American Society for Bariatric Surgery recently met in Memphis, where members paraded in the pants of formerly obese patients who have had the weight-loss surgery. (Rollin A. Riggs for The New York Times)(pg. A26) Chart: ''Losing Weight Through Surgery'' More people considered to be ''morbidly obese'' are turning to procedures known as bariatric surgery to lose weight as a remedy of last resort. GASTRIC BYPASS (MOST COMMON TECHNIQUES) The stomach is reduced to a small pouch, which restricts food intake, and the upper portions of the small intestine are bypassed to reduce calorie and nutrient absorption. ROUX-EN-Y GASTRIC BYPASS (Far more common, 2.5-5 feet of small intestine bypassed) Most of the stomach is separated from a small pouch and stapled closed. The duodenum and part of the jejunum are bypassed. EXTENSIVE GASTRIC BYPASS (Less common, 12-15 feet of small intestine bypassed) Portions of the stomach are removed to create a small pouch that is directly connected to the final segments of the small intestine. The duodenum is completely bypassed and sealed off. (Source: National Institutes of Health)(pg. A26)