Dr. Kelly D. Brownell, the director of the Yale Center for Eating and Weight Disorders, said the number was first suggested in a 1959 clinical study of only 100 people. The finding was repeated so often that it came to be regarded as fact, he said.

Since then, nearly all studies of weight-loss recidivism have followed patients in formal hospital or university programs, because they are the easiest to identify and keep track of. But people who turn to such programs may also be the most difficult cases, and may therefore have especially poor success rates.

To get a more accurate picture, two researchers are studying long-term dieters for a project called the National Weight Control Registry, and have found it surprisingly easy to collect success stories. About half the people who maintained a substantial weight loss for more than a year had done it on their own, they found. This suggests that many people have found ways to lose weight and keep it off, but have never been counted in formal studies.

''There is something very optimistic about this whole data set,'' said Dr. Rena Wing, a professor of psychiatry at the University of Pittsburgh and the Brown University School of Medicine. ''Without much effort, we have identified 2,500 people who have succeeded.''

Dr. Wing and Dr. James O. Hill of the University of Colorado are collaborating on the registry project, which they began five years ago with financing from drug companies and other sources. They are compiling detailed histories of successful long-term dieters -- people who had maintained a weight loss of at least 30 pounds for at least one year. Most of the volunteers were solicited through articles in newspapers and magazines. ''It was really to convince ourselves and convince the world that there are people who are successful, and then to learn from them,'' Dr. Wing said.