He estimated that about 8 percent of American teenagers would qualify.

“These are not kids who are pleasantly plump,” said Dr. John Morton , a bariatric surgeon at Yale University. “Once you have a B.M.I. of 30, it is really difficult to lose weight on your own.”

There is no other treatment that results in a substantial and sustained weight loss in people who are severely obese. But the operation, gastric bypass surgery, is demanding. Surgeons close off most of the stomach, leaving a small pouch, and reroute the intestines. Afterward, patients must eat tiny meals at frequent intervals for the rest of their lives.

It is a scenario that gives many parents pause. Should their teenagers wait, hoping science will come up with a less drastic solution? Or should their children have the operation before even more serious harm to the body occurs?

Adding to the quandary is the fact there are just six accredited bariatric surgery centers in pediatric hospitals. Although some adult programs operate on teenagers, most of such procedures are done in pediatric settings, Dr. Morton said.

Adults, by contrast, may choose from 850 accredited medical centers for weight-loss surgery.

Although the new study included adolescents with B.M.I.s of 35, most were much heavier. The average was 50, the same as the average for adults in the study.

For most of the adolescents, the surgery was a success. On average, they lost about a quarter of their body weight, enough to make life much easier and for most to shed health problems.

The teenagers weighed 324 pounds on average when they had the surgery. Five years later, the figure was 244 pounds. The adults weighed the same at the start and had a nearly identical result .