Every 30 minutes, all of the blood in our bodies is filtered through two fist-size kidneys. But diseases such as diabetes can cause them to fail, leading to a build-up of chemicals in the blood that without dialysis (mechanical blood filtration) or a kidney transplant would be fatal. And the wait for a new kidney can be long, unless someone you know is willing to give one of theirs to you.

Each year, more than 6,000 healthy people in the U.S. donate a kidney to someone they know. About half are blood relatives of the kidney recipient. The other half are spouses, friends or acquaintances. But about 100 come forward wishing to anonymously give the gift of glomeruli (the basic filtration units of the kidney) to someone they've never met.

"All these folks are heroes making a big difference in someone else's life," says transplant surgeon Dorry Segev, describing the willingness of donors "to undergo an operation which carries a risk and gives them no benefit." You only need one kidney to live. But the operation required to remove a kidney and the risk of developing disease in the remaining one later in life make the decision to donate not one to be taken lightly. "It's our job to quantify the risks and make sure those risks stay as small as they can," Segev says.

Segev and his team from Johns Hopkins University School of Medicine in Baltimore studied mortality among 80,000 kidney donors over the past 15 years, comparing them to healthy people with both kidneys. The study, published March 10 in JAMA The Journal of the American Medical Association—60 years after the first kidney transplant in the U.S.—found no increase in mortality in donors once they recover from the operation. "With this study we're able to say that whatever happens to people physiologically after kidney donation—it doesn't cause a premature death," Segev says. "After donating a kidney a person can live exactly the way they lived before donating—a long, healthy, active life with virtually no restrictions at all."

Whereas the mortality rate was no different among donors and healthy people one year after donation, three in 10,000 donors died within 90 days after surgery—a surgical mortality rate one sixth that for gall bladder removal and one fortieth that for nondonor kidney removal. Males, African-Americans and sufferers of hypertension were at a higher risk for postoperative death—a finding that Segev says is consistent with the general risk of surgery.

Segev hopes that the study will make people more aware of the major problems surrounding kidney disease, such as the profound organ shortage, wait-list mortality, and the poor quality of life for people on dialysis. "I hope that those things will inspire donors, and that our study will help quantify the risks so they can make intelligent choices," Segev says. Although donors are carefully screened before the procedure, he stresses that there are risks. "It's still a major operation. You're still living with one kidney. People still need to think about it and be aware of the risks in taking on this heroic act," he says. Donors are advised to see a doctor once a year after removal, in hopes that any trouble (like disease in the remaining kidney) would be caught early.