One hundred one carefully screened morbidity obese patients underwent jejunoileal bypass and were followed closely over a mean follow-up period of 32 months. Although there were no operative deaths, three per cent of patients died postoperatively of liver failure or its complications. A fourth patient died of a pulmonary embolus after reoperation, and the fifth patient died cachectic with severe diarrhea after excessive weight loss. Nineteen per cent of the patients required restoration of intestinal continuity (reversal), most for either liver failure or late fluid and electrolyte derangements. All but two survived reversal and are doing well despite massive weight gain. Fifty-eight per cent of the patients had major complications which either required major reoperation (reversal, cholecystectomy or incisional hernia repair) or were potentially life-threatening (liver failure, hepatic fibrosis or urinary tract stones). As described in other series, abnormalities in serum electrolytes and vitamins were seen. In addition, hypovitaminosis D occurred in a number of patients and as with other serum parameters measured, was time-dependent in that improvement was seen in most patients over the postoperative interval studied. Because of the high rate of complications and reversals, we believe that jejunoileal bypass should be reserved for patients with morbid obesity whose lives are imminently threatened by obesity or its sequellae.