More than 120,000 obese Americans had some kind of surgery to help them lose weight in 2004, with the biggest increase among middle-aged people, according to a study released on Wednesday.

In 1998, 772 people aged 55 to 64 had gastric bypass, stapling or some similar procedure known as bariatric surgery to help weight loss. But that number ballooned to 15,086 in 2004, according to the Agency for Healthcare Research and Quality.

“Among the reasons for the extremely dramatic increases is that the mortality outcomes from obesity surgery have improved greatly,” the agency said in a statement.

“The national death rate for patients hospitalized for bariatric surgery declined 78 percent, from 0.9 percent in 1998 to 0.2 percent in 2004.”

Agency director Dr. Carolyn Clancy said the report shows “more Americans are turning to obesity surgery and that an increasing number of younger people are undergoing these procedures.”

More than 103,000 of the 2004 operations were on patients aged 18 to 54, the study found, and 349 were performed on youngsters aged 12 to 17.

“As the rate of obesity continues to climb, the health care system needs to be prepared for continued escalation in the rate of this surgery and its potential complications,” she said.

The average hospital cost for an obesity surgery patient stay, excluding physician fees, was $10,395 in 2004 as compared with $10,970 in 1998, adjusted for inflation, according to the report, published online at http://www.hcup-us.ahrq.gov/reports/statbriefs/sb23.pdf.