The clinical, laboratory and histological features of 47 patients with what is defined as late onset hepatic failure are reviewed. Twenty-five of the patients were female and 22 male with a median age of 45 years. Hepatic dysfunction was severe as evidenced by the prolongation of prothrombin time (median = 32 sec, range = 17 to 120 sec). In only four cases was a viral etiology proven (2 hepatitis B, 2 hepatitis A) although the similarity of the clinical features to patients with fulminant viral hepatitis--apart from the longer period of illness prior to the onset of encephalopathy (median = 9 weeks, range = 8 to 24 weeks)--made non-A, non-B infection a possibility in the remainder. There were also similarities to chronic active hepatitis with low titer antibodies to smooth muscle or antinuclear factor in 17% and elevation of the serum IgG in 49%. Liver biopsy in 5 of 8 survivors more than 1 year after initial presentation showed chronic active hepatitis in three. Lobular inflammatory infiltrate, bridging necrosis and multilobular collapse were the features of the acute stage of illness in both the survivors and fatal cases. The patients given corticosteroids did not have a statistically significant improvement in survival, and overall mortality for the series was 81%. Hepatic transplantation, successfully performed in one patient, would appear to offer the best chance of survival for the majority of these patients.