Obesity is associated with increased morbidity and mortality. Surgery for morbid obesity may be considered when other conventional measures have failed, and a number of procedures are available. However, the effects of these surgical procedures compared with medical management and with each other are uncertain.To assess the effects of surgery for morbid obesity on weight, comorbidities and quality of life.We searched the Cochrane Controlled Trials Register (issue 4, 2001), Medline (SilverPlatter) up to 2001, PubMed (Internet) 01/01/01-19/10/01, Embase (SilverPlatter) up to 09/2001, PsychINFO up to 10/2001, CINAHL (SilverPlatter) up to 07/2001, Science and Social Sciences Citation Index up to 10/12001, British Nursing Index up to 07/2001, Web of Science Proceedings up to 06/2001, BIOSIS up to10/2001, AMED up to 07/2001, National Research Register (issue 2, 2001), reference lists of relevant articles, and handsearched relevant journals. We also contacted experts in the field. Date of the most recent searches: October 2001.Randomised controlled trials comparing different surgical procedures, and randomised controlled trials and non-randomised controlled trials comparing surgery with non-surgical management for morbid obesity.Data were extracted by one reviewer and checked independently by two reviewers. Two reviewers independently assessed trial quality.18 trials involving 1891 people were included. One randomised controlled trial and one non-randomised controlled trial compared surgery with non-surgical management, and 11 randomised controlled trials compared different surgical procedures. The overall quality of the trials was variable, with just one trial having adequate allocation concealment. A meta-analysis was not possible due to differences in the surgical procedures performed, measures of weight change and length of follow-up. Compared with conventional management, surgery resulted in greater weight loss (23-28 kg more weight loss at two years), with improvements in quality of life and comorbidities. Some complications of surgery occurred, such as wound infection. Gastric bypass was associated with greater weight loss and fewer revisions, reoperations and/or conversions than gastroplasty, but had more side-effects. Greater weight loss and fewer side-effects and reoperations occurred with adjustable gastric banding than vertical banded gastroplasty, while vertical banded gastroplasty was associated with greater weight loss but more vomiting than horizontal gastroplasty. Some postoperative deaths occurred in the studies. Weight loss was similar between open and laparoscopic procedures. Fewer serious complications occurred with laparoscopic surgery. Laparoscopic surgery had a longer operative time, but resulted in reduced blood loss, reduced proportion of patients requiring intensive care unit stay, reduced length of hospital stay, reduced days to return to activities of daily living and reduced days to return to work.The limited evidence suggests that surgery is more effective than conventional management for weight loss in morbid obesity. The comparative safety and effectiveness of different surgical procedures is unclear.