By the time Linda Culpepper found her way to the Vanderbilt University Medical Center, she was in an alarming state. Her hair was falling out, her skin was flaking, and her muscles had wasted so much that it was hard for her to walk. She had frequent attacks of diarrhea, and could rarely eat without vomiting.

"She was a shadow of a human being," said her daughter, Susan Gritton.

Dr. Gordon L. Jensen, director of the Vanderbilt Center for Human Nutrition in Nashville, diagnosed her condition as life-threatening malnutrition, admitted her to the hospital and ordered intravenous feeding immediately.

The cause of the malnutrition was complications from weight-loss surgery performed at another hospital, specifically a gastric bypass, a procedure that closes most of the stomach and shortens the small intestine, often leading to weight losses of 100 pounds. That is the operation that has strikingly transformed celebrities like Al Roker, the television weather forecaster; Carnie Wilson, the singer; and Roseanne Barr, the comedian.

Successful cases like theirs, combined with a growing epidemic of obesity, have led to soaring demand for the surgery. In 1995, just 20,000 weight-loss operations were performed in the United States. Last year, there were 103,000, and this year 144,000 are projected.

The surgery has become big business, and medical centers have been scrambling to start programs.

The rapid growth worries experts like Dr. Jensen, as well as some insurers and government officials, who fear that inexperienced surgeons and inadequate screening and follow-up may harm patients.

In the last year, Dr. Jensen said, he has seen a "tremendous surge" in patients like Ms. Culpepper who have complications from the surgery or have not been taught how to change their eating habits to adjust to the drastic changes in their digestive systems. Most of the patients had surgery at smaller hospitals that were not equipped for the problems, he said, adding that he sees as many as one such case a week.

A recent study suggests that the overall death rate is twice the figure of 0.5 to 1 percent that is usually cited, and higher still if a surgeon lacks experience.

Researchers also express concern that the operations are being performed on children and teenagers with scant evidence about long-term effects. In addition, although the surgery is supposed to be limited to people who are 100 pounds or more overweight, Dr. Jensen and other experts said some doctors had actually encouraged obese patients who were not heavy enough to gain more weight so they would qualify.

Dr. Philip Schauer, director of bariatric surgery, the technical name for weight-loss surgery, at Magee-Womens Hospital of the University of Pittsburgh, which has one of the largest programs in the United States, said: "There are a lot of surgeons who are new to this field and frankly haven't had much training. It's the biggest problem we're having right now in this field."

In an interview last month, Dr. Schauer said, "I've got three patients now that were treated by other surgeons, with major complications."

To master a weight-loss operation, he added, a surgeon needs to perform it 100 times.

Any surgeon can perform weight-loss surgery if a hospital will allow it. Although a professional group recommends that weight-loss surgeons meet certain criteria, its guidelines are not binding.

Citing dangers and increased costs, some insurers have decided to stop covering the operations. In February, Blue Cross Blue Shield of Florida called the operations extremely risky, questioned their benefits and said it would stop paying for them after this year.

Dr. Barry Schwartz, a vice president of the Florida plans, said: "Folks now doing them and the hospitals doing them I think are questionable. The physicians are increasingly folks with little experience and remarkably little training, down to and including weekend courses."

A gastric bypass usually costs $30,000, but can cost much more if serious complications occur. The Florida insurer's spending on weight-loss surgery nearly doubled in the last two years, to $17 million a year, and it projected the sum to run $200 million over the next three to four years, a spokesman said.

In Massachusetts in February, the State Health Department convened an expert panel to study the surgery because three patients died last year during or after operations at different hospitals in Boston. The panel is to issue a report next month and recommend ways to make the surgery safer.

The National Institutes of Health has also begun a study of the surgery. And the professional group for doctors who perform weight-loss surgery, the American Society for Bariatric Surgery, has begun a program to identify "centers of excellence" for the operations, collect information on their results and use it to help others adopt the best surgical techniques.

In the meantime, victims of botched operations or poor follow-up care continue to turn up.

Dr. Jensen said, "One of the key things we're seeing and find quite disconcerting is that a lot of the places where the programs are popping up don't have the evaluation and education components in place pre- or postoperatively."

At the Tufts New England Medical Center in Boston, Dr. Scott Shikora, an obesity surgeon, said he had seen one or two dozen patients with complications in the past few years, referred from other centers, usually smaller hospitals. "If you ask any major medical center, you'll hear the same story," Dr. Shikora said. "They are receiving patients who were mismanaged."

Weight-loss operations are challenging even for the best surgeons, and the enormously obese who seek them are often high-risk cases because of diabetes or heart or lung problems. Because the surgery makes such drastic changes in the digestive system, patients need extensive counseling about how to eat and take vitamins, or they risk malnutrition or weight gain.

When conducted properly, Dr. Jensen said, the surgery can give patients a new lease on life, and for many it is the only thing that works. It can restore health, as well as appearance, quickly easing conditions like diabetes and high blood pressure.

Like any surgery, it has risks. The most common serious complications include bleeding, blood clots, bowel obstructions, hernias and severe infections from leaks in the belly where the stomach and small intestines have been cut and sealed or reconnected.

According to the National Institutes of Health, 10 to 20 percent of patients need additional surgery for such complications, and nearly 30 percent develop nutritional deficiencies that lead to conditions like anemia and bone loss.

"And usually, the best results have very little to do with what's going on in the community at large," he added. "It is particularly important that we acknowledge that the risk of death is higher than previously reported. Nobody's looking at new centers out there and their mortality rates."

Patients who survive the surgery do live longer than very obese who do not have the surgery, Dr. Flum's study found. After 15 years, patients who had the bypass were more likely to be alive than those who did not, 88.2 percent versus 83.7 percent.

The difference was greater in patients younger than 40. Just 7.6 percent of those patients had died, compared to 15.9 percent of those who did not have the operation.

When the researchers compared survival rates starting at one year after the operation, they found that patients who had surgery were 33 percent less likely to die than those who did not.

Some obese people are extremely ill because of their weight, with heart and lung disease, Dr. Flum said. For them, he said, a 2 percent mortality rate from surgery may be a reasonable risk.

All of the several types of bariatric surgery can be performed as an open procedure or through a laparoscope, which requires a few punctures instead of an incision. Specialists call the gastric bypass "the gold standard."

More extreme operations can bypass more of the intestine and close off less of the stomach. Surgeons call one of the more drastic procedures the "duodenal switch."

Dr. Ostroff said many patients had been misled into thinking that their bypasses would solve their weight problems forever without any effort on their part. That is not true, he said. Over time, the pouch stretches, and if people do not exert some self-control, they will regain weight.

"We feel the majority of individuals will gain the weight back over about 20 years," he said, noting that a gain of 5 pounds a year was not uncommon.

Ms. Culpepper, now 60, had a bypass in January 2003 at a small hospital in Georgia. Five feet tall, she weighed 258 pounds and had diabetes and high blood pressure. She wanted the surgery because she was convinced that her obesity would kill her.

She had just one meeting each with a dietitian, a psychologist and a cardiologist before the surgery, and there was no support group, she said.

When Ms. Culpepper was finally released, without the vitamins essential after surgery, she had diarrhea much of the time and vomited so much that she could barely eat. Her medical bills, paid by Medicare and Medicaid, ran more than $1 million.

She now weighs 127. "Right now, I'm on top of the world," she said. "I'm happy, I'm still fairly healthy right now."

Nonetheless, Ms. Culpepper said, she wishes she had never had the operation. She said, "I think I've been able to talk a few people out of it."