WASHINGTON — Stomach surgery to treat obesity, which is soaring in popularity, is much riskier than had been thought, with patients facing a far greater chance of being hospitalized and dying after the 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 Flum of the University of Washington, 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 outweigh the risks for many patients, and efforts are under way 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 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 number of people undergoing stomach surgery has been rising quickly, jumping from just over 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.

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. Among patients 65 to 74, nearly 13 percent of men and about 6 percent of women died. In patients 75 and older, half of the men and 40 percent of the women died.

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

Researchers said one reason men may have higher post-surgery death rates is that they tend to wait longer than women to seek medical help and may be sicker when the operation is performed. 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.

In the second study, David 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.

Material from The Associated Press is included in this report.