Fifty-eight patients less than 30 years old but who are more than 300 pounds in weight were considered for investigation of jejunoileal bypass. Thirty patients had operations, and the functioning bypass included 40 centimeters of jejunum and 10 centimeters of ileum. In 14 patients, intestinal continuity was restored by either end-to-end or end-to-side anastomosis according to random choice, and in 16 patients, end-to-end anastomoses were used. At present, the follow-up period is one to four years, and the weight loss, so far, has not been significantly related to the type of reconstruction used. Weight loss correlated possitively with the preoperative weights up to one year after operation. Also, weight loss correlated positively with the total measured length of the small intestine during the first six postoperative months, but this correlation currently is showing a trend toward negativity. Food intake decreased by 2,682+/-690 calories per day at six months after operation as a result of the bypass. Chronic acidosis was common. Results of an endocrine evaluation revealed an unexpected significant decrease in parathormone levels within the normal range. A new complication, colonic pseudo-obstruction, has occurred one year or more after operation in five patients or 17 per cent of our group. This complication is related to the intestinal anaerobic flora in proximity of the defunctionalized limb; its symptoms and signs can be alleviated by giving antibiotics or Lactobacillus to change the intestinal flora. Although 90 per cent of our patients are pleased with their progress and the results of routine evaluation corroborate their satisfaction, detailed analysis of the research data available to us revealed that only 43 per cent have had beneficial effects from a jejunoileal bypass without any of the serious sequelae. We consider jejunoileal bypass for obesity justified when carried out by interdisciplinary groups dedicated to the long term follow-up study of the patient and periodic reporting of the results. The long term future for the operation is, as yet, not completely understood, and when the conditions we have recommended are not present, we cannot currently endorse the operation.