Stomach surgery to treat obesity is much riskier than had been thought, with patients facing a far greater chance of being hospitalized and dying following the increasingly popular operations, according to two large new studies.

One analysis of more than 60,000 California patients found they were twice as likely to require hospitalization after the operations than before, while the second study of federal data from more than 16,000 patients nationwide found the chance of dying after being released from the hospital was significantly higher than earlier studies indicated.

The studies are the largest to examine the risk of hospitalization and death after the surgery, which helps obese people lose weight by reducing the size of their stomachs.

"This is a reality check for people thinking of having obesity surgery," said David R. Flum of the University of Washington at Seattle, who led the nationwide study. "Patients really need to go into this with their eyes open. This can be a high-risk operation."

Proponents of the operations said the benefits still surpass the risks for many patients, and efforts are underway to make the procedures safer by more carefully selecting appropriate patients and ensuring the procedures are done by the most experienced surgeons.

"We're talking about patients for whom nothing else works," said Harvey J. Sugerman of the Virginia Commonwealth University in Richmond, speaking on behalf of the American Society for Bariatric Surgery. "They can't walk. They can't work. They are miserable. They have diabetes and hypertension, and they die at a much higher rate. This operation is the only thing that helps these people."

The new studies come as the federal Medicare program is considering whether to pay for the procedures nationwide, a move that proponents of more aggressive medical treatment for obesity see as crucial for encouraging insurance companies to cover more weight-loss therapies in response to the nation's obesity epidemic.

Critics say the new research should dissuade the government from covering the procedures.

"Expansion of Medicare to cover weight-loss surgery will result in numerous malnourished and frail elderly, many of whom will die prematurely as a result of this treatment," said Paul Ernsberger of Case Western Reserve School of Medicine in Cleveland.

Steve Phurrough of the Centers for Medicare and Medicaid Services said officials would consider the new data in making the decision, which is expected by the end of November.

"The question that you really want to answer is, 'Does it work better than not having the surgery?' This research makes me -- and should make anyone who wants to have the surgery -- concerned about doing the surgery until we see data that says not having the surgery makes you more likely to die," said Phurrough, who heads the office that is evaluating whether to cover the procedures.

The number of people undergoing stomach surgery has been rising quickly, jumping from more than 13,000 in 1998 to nearly 103,000 in 2003, according to a third study published in today's issue of the Journal of the American Medical Association. If current trends continue, the number will surpass 130,000 this year and could hit 218,000 by 2010, the researchers projected.

Proponents say the operations not only help obese patients lose significant amounts of weight, but also reverse many of the most serious obesity-related health problems. The safety of the procedures, however, has been the focus of intense concern, with reports of deaths and complications sparking investigations in several states and some insurers balking at paying for the operations.

To gather more accurate safety information, Flum and his colleagues analyzed data collected by Medicare on 16,155 patients who underwent the surgery from 1997 to 2002. The average age of the patients was 48.

Overall, 2 percent of patients died in the first month after surgery, nearly 3 percent died within the first three months and nearly 5 percent died within the first year, the researchers found. Previous reports have generally put the risk well below 1 percent. The risk was higher among men, and much higher among those 65 and older, putting those patients at nearly double the risk they would face from heart bypass and hip surgery, Flum said.

Most deaths result from infections or the stress of surgery, Flum said.

The procedure's supporters said the findings could be explained at least in part by the fact that Medicare patients tend to be older and sicker than the general population.

In the second study, David S. Zingmond of the University of California at Los Angeles and his colleagues analyzed data from 60,077 people who underwent surgery in California from 1995 to 2004. They found that about 8 percent were hospitalized in the year before the surgery, but nearly 20 percent were hospitalized in the year afterward.

Most of the hospitalizations were for complications from the surgery, such as nausea and dehydration, gallbladder problems or plastic surgery to remove excess flab.

The American Society of Bariatric Surgery has launched a program designed to minimize the risks by standardizing the procedures and certifying centers that have the most experienced surgeons, who tend to have better outcomes.

"This is the best way to get at the problems that these papers raise," said Walter Pories of East Carolina University in Greenville, N.C. "There's no question that the mortality in some cases is higher than we would like, but it's still less for many of these patients than if they didn't have the surgery."