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.

To their surprise, Dr. Wing and Dr. Hill found that on average the respondents had maintained a 67-pound weight loss for five years. Between 12 and 14 percent had maintained a loss of more than 100 pounds.

The success stories might not have been so surprising had it been clear that the 95 percent failure rate was so poorly founded. The figure comes not from any kind of random sampling, but from a study of 100 patients treated for obesity at a nutrition clinic at New York Hospital in the 1950's. In 1959, its authors, Dr. Albert Stunkard and Mavis McLaren-Hume, published a paper in which they concluded, ''Most obese persons will not stay in treatment, most will not lose weight, and of those who do lose weight, most will regain it.''

That conclusion, Dr. Brownell of Yale said, has since become the most frequently quoted statement in obesity literature.

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Dr. Stunkard, who is now a professor of psychiatry at the University of Pennsylvania, said the study was ''perfectly respectable'' for that period. ''The paper made a big impact because everybody thought obesity was pretty easy to treat,'' he said. ''This showed that, for whatever reason, it wasn't.''

But the study has little relevance to the current understanding of how to control weight, said Dr. Stunkard, who specializes in the treatment of obesity and eating disorders. The 100 patients in the study were ''just given a diet and sent on their way,'' he said.

''That was state of the art in 1959,'' he added. ''I've been sort of surprised that people keep citing it; I know we do better these days.''

The intervening years have brought significant changes to the treatment of obesity, the most important of which, Dr. Stunkard said, has been the introduction of behavior modification techniques.

Since the 1959 study, though, the statistic has been reinforced by most other clinical studies, which also showed people with discouraging results.

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''Unless we can prove they're typical, the data cannot be generalized,'' Dr. Brownell said. ''The people we see in clinics tend to be more overweight and have more psychological problems. They are more likely by a factor of two to have binge-eating problems.''

The true failure rate could be much better, or much worse, he said. ''The fact is that we just don't know.''

The new research on successful dieters will disappoint those hoping for a magic-bullet solution; most had simply eaten less, and healthier, food, and exercised regularly. But judging by their accounts, it is entirely possible for people without the resources to hire personal trainers and chefs to accomplish permanent weight loss.

For inspiration, it would be difficult to surpass the story one of the participants in the Weight Control Registry, Tammy Munson, a 32-year-old receptionist and ambulance dispatcher from Jamestown, N.Y., who lost 147 pounds and has kept it off for eight years.

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At 5 feet 4 inches tall and 185 pounds, she was overweight throughout high school, but ballooned to 253 after she graduated and got married. She knew little about nutrition, she recalled, and ''didn't even realize that liquid had calories.''

''I'd get up in the morning and have ice cream and cookies,'' Ms. Munson said. Dinner might be three or four fast-food hamburgers and candy bars. Her husband, Jeffrey, who has lost 100 pounds, was also seriously overweight. ''Our wedding pictures are really funny,'' she said. ''We broke the bed when we were first married.''

Ms. Munson began to face her weight problem, she said, when her blood pressure began to rise. At first she did some crash dieting, eating only about 650 calories a day.

''I read every book in the library about how to keep the weight off,'' she said, and eventually settled on a more balanced approach that included salads, fish, vegetable burgers, fruit and always a little dessert. And exercise. She exercises to a tape every day, jumps rope, and has begun running.

In part, Ms. Munson attributes her success at least in part to the fact that she did not know success was supposedly impossible.

''You've got to find it for yourself,'' she said. ''And don't spend money on it. You can get it for free. You can get if from reading. It's just the food pyramid, but people don't want to hear it. Slimfast, Optifast, you can't live like that forever.

''You eat a lot of good stuff and a little bad stuff, and you'll be fine, but you've got to have the little bad stuff, too.''

The experience of Russell M. Lomando, 47, of Brooklyn, was less dramatic, but probably more typical of the people in the study. ''I was 39 years old and I was walking up the train stairs in Brooklyn huffing and puffing,'' he said. ''I could see myself going into the grave when I was 40.'' He used to eat, he said, ''pounds of junk -- donuts and cookies and cakes, and I'm talking boxes at a time.''

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One day, climbing the subway stairs, Mr. Lomando made up his mind to cut out the junk, but gradually.

At first, he was able to do it only four days a week; he worked up to seven over a period of months. He took up karate for exercise, and over two years lost 50 pounds, getting his weight down to 160.

''You just have to change your life style,'' he said. ''You just have to have the will power to do it.'' But that is not necessarily easy, he conceded. ''You just have to weigh the choices. Do you want to become healthier?''