The study of bariatric surgery outcomes at hospitals in 19 states from 2003 to 2005 also found that five-star rated hospitals -- those with better-than-average patient outcomes -- performed about twice the number of procedures compared with hospitals that rated poorly, according to the study released by HealthGrades, the healthcare ratings company.

The study also found a clear trend away from traditional, more invasive gastric bypass to a less invasive laparoscopic procedure, according to the second annual HealthGrades Bariatric Surgery Trends in American Hospitals.

More than 70 percent of the surgeries done in 2005 were laparoscopic, which are associated with fewer in-hospital complications than traditional gastric bypass.

"Bariatric surgery has been demonstrated to be highly effective for those with morbid obesity, but the relatively new procedures are not yet regulated or a credentialed surgical subspecialty," Dr. Samantha Collier, HealthGrades' chief medical officer, said in a statement. "So it is important that patients considering surgery know how hospitals rate."