Hepatic and renal failure developed in association with severe enteritis and hemorrhagic proctocolitis in a patient who had had a jejunoileal bypass 8 yr previously for morbid obesity. Parenteral antibiotic treatment abolished the systemic manifestations of the enteritis, but did not change the course of the hepatic and renal failure, and prolonged hemodialysis was necessary. Liver function improved in response to hyperalimentation. Take-down of the jejunoileal bypass resulted in immediate improvement of renal function, and hemodialysis could be discontinued. Although there is no direct evidence supporting this theory, the course of this patient suggested that the renal failure was functional in origin, and was caused by a toxin generated as a result of the intestinal bypass. We suspect that the toxin originated from bacteria within the blind bowel loop. Its delivery to the renal circulation was probably facilitated by increased absorption from the ulcerated large intestine and by impaired clearance by the diseased liver. When the bacterial flora were returned toward normal by take-down of the bypassed intestine, the quantity of circulating toxins probably decreased, which allowed renal function to improve.