To assess the pattern and severity of liver disease in patients who had undergone jejuno-ileal bypass for the treatment of morbid obesity, 23 patients were biopsied, all of whom had had intact bypasses for more than 10 years. These were examined by light and electron microscopy. Previous biopsy specimens from each patient, including specimens taken before bypass, were reviewed. Similar biopsy specimens were obtained from six obese patients undergoing gastroplasty. There was no evidence of cirrhosis, and mild or moderate degrees of fibrosis were found in only a few patients. Steatosis tended to persist after bypass, albeit to a lesser degree. Giant mitochondria and intramitochondrial filamentous inclusions were present in four of six specimens taken before gastroplasty and in 15 of 23 specimens taken after jejuno-ileal bypass, being especially numerous in those specimens showing little or no steatosis. It is suggested that such features reflect ultrastructural evidence of adaptation to an abnormal metabolic environment both in the morbidly obese and even many years after jejuno-ileal bypass. Their prognostic importance is unclear.