Intestinal bypass surgery, performed for weight reduction in the morbidly obese patient, is frequently complicated by the development and hepatic complications. In 44 morbidly obese individuals, 55 inches of proximal jejunum were anastomosed, end to side, to 5 inches of distal ileum. All the patients were followed with body composition measurements, performed by multiple isotope dilution, prior to and at regular time intervals following bypass surgery. In 33 patients a decrease in body fat accounted for the entire postbypass weight loss, while the lean body mass remained normal in both size and composition. In these patients, at 1 year, body weights had decreased by 24.4 +/- 2.1%, while the body cell masses had decreased by 2.1 +/- 7.1%. In the remaining 11 patients, the postbypass weight loss resulted from a loss of both body fat and body cell mass. Their body weights at 1 year had decreased by 27.0 +/- 3.0%, while the body fat and body cell mass. Their body weights at 1 year had decreased by 27.0 +/- 3.0%, while the body cell masses decreased by 22.0 +/- 6.1%. Furthermore, their body compositions were characteristic of protein malnutrition with a contracted body cell mass and an expanded extracellular mass. Six of these 11 patients have required admission to hospital on 10 occasions because of malaise, anorexia, debilitating weakness, hypokalemia, and abnormal liver function. They were treated for 14.5 +/- 1.9 days with an intravenous infusion of amino acids without additional nonprotein calories. The body composition, initially characteristic of malnutrition, became normal. Their symptoms disappeared and hepatic function returned to normal. Subsequently a high-protein diet was required to prevent a recurrence of symptoms and to maintain a normal body composition. The data indicate that protein malnutrition developed in 11 of 44 patients undergoing jejunoileal bypass for weight reduction.