This report details the complications observed in 154 morbidly obese patients who has a gastric bypass performed in the period from April 1975 through March 1979. Thirteen intraoperative complications occurred. In the first 30 days after operation, one or more complications developed in 29 or 17.9 percent of the patients. One patient died from pulmonary embolism. The most serious complication unique to gastric bypass was a leak from the stomach or the anastomosis. The factors responsible are discussed and suggestions are made concerning the prevention of this complication. Sixteen late complications occurred in 13 patients. The 62 operative procedures performed incidental to gastric bypass and the 50 performed subsequently as a result of it are enumerated and discussed. The findings of this investigation document the serious complications that may occur with gastric bypass. The late complications of this operation are few and trivial compared with those that occur with jejunoileal bypass [1,2,4,5,9].