Detailed postmortem examination was carried out on five patients who died three months to four years after jejunoileal bypass for obesity. A spectrum of histological changes was observed in the liver, with pericentral fat deposition being a common feature. Evidence of previous and/or ongoing liver cell dropout with accompanying polymorphonuclear and mononuclear infiltration was seen in all cases, but Mallory hyalin was not detected. Liver function abnormalities included decreased plasma protein levels, decreased prothrombin activity, increased serum alkaline phosphatase levels, and variable elevations of the serum transaminases, bilirubin, and ammonia concentrations. The pattern of the hepatic disease does not resemble protein deficiency. An uncharacterized hepatotoxin or toxic effect of hepatic fat accumulation may play a significant role in the changes observed in these patients.