Clinical profiles, serological markers, and antibody responses to antigens of hepatitis B virus (HBV) were studied in patients with fulminant viral hepatitis. Whereas hepatitis A and B were found to be uncommon causes (6.9% and 12.2%, respectively), non-A, non-B (NANB) hepatitis was found to be the most common cause of fulminant hepatitis (80.9%). As against this, the incidence of hepatitis B and NANB hepatitis was very similar in nonfulminant acute viral hepatitis in adults (41.2% and 51.9%, respectively). Pregnancy with labour was an important precipitating factor for development of fulminant hepatitis of the NANB type only; 32% of fulminant NANB hepatitis patients were pregnant women and 22.6% had a history of labour preceding hepatic coma. Only 0.8% of nonfulminant NANB hepatitis cases were pregnant women. Another major precipitating factor for the development of the fulminant form of NANB hepatitis was concomitant chronic HBV carrier state. A total of 38.6% of fulminant NANB hepatitis patients were HBV carriers, whereas only 19.2% of nonfulminant acute NANB hepatitis cases were HBV carriers. Sera of 32 chronic HBV carriers with fulminant NANB hepatitis and 10 cases of fulminant hepatitis B were tested for delta antibody, and all were nonreactive. The antibody responses of the fulminant hepatitis B patients to the antigens of HBV were found to be greater compared to those of patients with nonfulminant acute hepatitis B. Antibody responses of chronic HBV carriers with fulminant NANB hepatitis to antigens of HBV were found to be depressed in comparison with those of chronic asymptomatic carriers.