Twenty-one female patients studied between six and 12 months following a jejunoileal bypass procedure for obesity were found to have a wide variety of metabolic disturbances. Hepatic histological abnormalities were common and included liver cell necrosis and inflammation in nine patients and hepatic fibrosis in five. Liver function tests were no guide to the degree of hepatic impairment. Vitamin B12 malabsorption occurred in seven patients, in six probably as a result of bacterial intestinal overgrowth; three of these six patients had the most serious hepatic morphological changes. Malabsorption rather than poor oral intake of food appeared to account for continued postoperative weight loss in the majority of patients.