The adjectives fulminant and subfulminant apply to those forms of acute hepatitis where the prothrombin level falls below 50 p. 100 of its normal value and which is complicated by clinical hepatic encephalopathy. The most frequent cause is an acute viral hepatitis, notably that due to the B virus. Beside jaundice and encephalopathy, the most striking manifestations consist of low factor V level, cardiocirculatory hyperkinesia, acute renal failure due to hypovolaemia, respiratory alkalosis and sometimes hypoglycaemia or hypophosphataemia. The mortality rate, which is 75 p. 100 overall, varies with the causative virus. The prognosis is based on the degree of factor V decrease rather than on the severity of the encephalopathy. Coagulant fractions and nervous sedatives may obscure the prognosis and should not be administered. With emergency liver transplantation, 60 p. 100 of the patients who would have died survive. Early hospitalization of patients with severe acute hepatitis (prothrombin level below 50 p. 100 of its normal value, but no hepatic encephalopathy) is associated with much lower mortality rates and could be used to prevent transformation into fulminant or subfulminant hepatitis.