To evaluate the results of jejunoileal bypass for morbid obesity, we studied 100 patients with intact bypasses an average of more than five years after surgery. Mean weight loss at five years was 102.7 lb (46.6 kg) (33 per cent). Although nearly half the patients regained some weight between one and five years after surgery, only 17 per cent regained 20 lb (9 kg) or more. Medical benefits (such as improved glucose tolerance and lowered blood pressure) were maintained at five years, but side effects and complications continued to occur in the late postoperative period. Diarrhea (more than three stools per day) persisted in 58 per cent of the patients, and electrolyte disturbances occurred in over a third. Diminished levels of B12 or folate or both were present in 88 per cent. Twenty-one per cent of the patients had nephrolithiasis, and 20 per cent of those who were at risk required cholecystectomy. Progressive hepatic structural abnormalities occurred in 29 per cent of the patients, and there was a 7 per cent incidence of cirrhosis. Although 81 per cent of the patients had satisfactory results at five years, side effects and complications continued to occur, mandating careful follow-up indefinitely. The risk-to-benefit ratio at five years after surgery seems acceptable, but the continued untoward effects of the bypass in the late postoperative period have led us to abandon this procedure in favor of gastric bypass. Only continued longitudinal follow-up will determine whether on balance jejunoileal bypass represents such a serious long-term health hazard that prophylactic restoration of intestinal continuity is indicated.