Dr. Mindy Rauch thought she was humoring her brother at a family gathering last Father's Day when she agreed to examine his 6-month-old daughter, Hannah. The baby's stomach worried him: it felt taut, and too big. Dr. Rauch, a pediatrician, thought her brother was just another nervous first-time father.

''Oh, look at this belly,'' she said playfully, feeling the baby's middle. A moment later she turned to her husband, who is also a pediatrician, and said, ''You have to feel this.''

Hannah's liver and spleen were enormously enlarged. It was a sign of serious illness: possibly a tumor, a viral infection or a genetic disorder.

For Hannah's parents, Scott and Jill Stimell, the next few weeks became a series of desperate trips to doctors and hospitals.

On July 16, the answer came: Hannah had a rare genetic disorder, Niemann-Pick disease type A.

It is hard to imagine a worse diagnosis. Infants with the disorder seem normal for the first few months, but the disease is already damaging the brain, the liver, the lungs and other organs. There is no treatment. The babies weaken, fade mentally and die, usually before they turn 3.

Within hours of learning Hannah's diagnosis, the Stimells faced an agonizing choice: they could watch her die, or pursue a drastic and painful treatment that had never been used successfully on a child with her condition. The treatment, chemotherapy and a transplant of umbilical cord blood, might save Hannah, do nothing or kill her. It also might save her body, but not her mind.

''Possibly we could save my daughter's life, but then she could be a complete vegetable for the rest of her life,'' Mr. Stimell said. ''Do I put my little baby through this hell knowing there's a great chance it won't work?''

And yet they felt it would be unbearable to stand by and do nothing.

''We felt like we had to take a shot,'' Mrs. Stimell said. ''She's a fighter. She has something in her.''

Niemann-Pick is one of about 50 metabolic disorders known as storage diseases, in which a substance the body should break down instead builds up and causes illness. In this case, a fatty molecule, sphingomyelin, accumulates because patients lack an enzyme needed to degrade it. There are several forms of the disease; type A is the most severe.

Why excess sphingomyelin causes so much damage is not clear, said Dr. Edward Schuchman, a professor of human genetics at the Mount Sinai School of Medicine in Manhattan. The molecule is a normal component of cell membranes, but too much of it may alter the membranes and disrupt cell function, Dr. Schuchman said.

The enzyme to break it down is missing because of a genetic mutation. For a child to be affected, both parents must carry the defective gene. Then there is a one in four chance of having an affected child with each pregnancy. But carriers have no symptoms.

Niemann-Pick, Tay-Sachs disease and several other genetic disorders are more common in Jewish people with ancestors from Germany, Poland, Austria or Eastern Europe -- Ashkenazi Jews -- than in other groups, and many Jewish couples seek genetic tests before having children. If both parents are carriers, the fetus can be tested, and if a devastating disease is found, some couples end the pregnancy.

The Stimells were stunned by Hannah's diagnosis because Mrs. Stimell had been tested for Niemann-Pick and told she was not a carrier. Later, a test showed that she carried a rare mutation not picked up by the standard test.

The day Niemann-Pick was diagnosed in Hannah, an uncle who is a pediatrician began searching the Internet and found that doctors at Fairview-University Children's Hospital, at the University of Minnesota, were treating storage diseases with transplants of bone marrow or umbilical cord blood. The family contacted Dr. Charlie Peters, who told them that the group had not been treating Niemann-Pick type A. In the past, they had tried bone marrow transplants in two babies with the disease, but the treatment failed and both died.

However, Dr. Peters said, the technology had advanced, and a transplant might now be worth considering. But the family would need to act fast, he warned, because the disease progresses rapidly.

A week later, on July 24, the Stimells, who live in Long Beach, N.Y., flew to Minneapolis.

Doctors began using bone marrow transplants to treat storage diseases in the 1980's. Performed early enough, the transplants can save children's lives and halt progressive brain damage. The University of Minnesota, which has treated several hundred children, is a leader in the field, along with Duke University, which has treated about 100, including some newborns.

Bone marrow contains stem cells that produce blood and immune-system cells. Blood from the umbilical cord and placenta, collected after a baby is born, is also rich in those stem cells, and in the 1990's doctors began using it for transplants.

Hundreds of thousands of cord blood samples have been frozen and banked, and are available immediately if a donor and recipient have matching tissue types. Bone marrow takes longer to acquire because it is not usually banked but is collected from donors only when it is requested for a matching recipient. Because Hannah has a progressive disease, her doctors hoped to use cord blood rather than a bone marrow donor so they could start treating her as quickly as possible.