Sixty-five patients with histologically proven chronic active hepatitis of unknown cause but associated with the antiliver/kidney microsome antibody type 1, confirmed by immunofluorescence and immunoprecipitation, were selected as forming a special entity. This disease was found to be rare with a prevalence of 6/1,000,000. The female to male ratio was 8:l. The condition occurred at all ages but was most common between the ages of 2 and 14 years. In 22 of the 65 cases, the hepatitis was associated with an autoimmune disease, most commonly insulin-dependent diabetes, autoimmune thyroid disease and vitiligo. The same autoimmune diseases were present in first-degree relatives from seven families. In 36 cases, the onset of disease resembled acute viral hepatitis. Serum biochemical tests showed marked elevation in aminotransaminases and hypergammaglobulinemia. Paradoxically, serum and salivary IgA levels were often normal or low. Histologic findings were multifocal hepatic necrosis with bridging in the acute stage, and aggressive hepatitis with mononuclear cell infiltration or macronodular cirrhosis in the late stages. Serologically, apart from the presence of antiliver/kidney microsome antibody type 1, the disease was characterized by the absence of antiactin, antimitochondria and antinucleus antibodies; however, organspecific autoantibodies were often present. The clinical course was usually severe: six patients in the acute stage presented with fulminant hepatitis, and all, except two, other patients progressed to cirrhosis, Prolonged treatment with corticosteroids and immunosuppressants was usually effective in rendering the cirrhosis inactive. The cumulative survival rate was 6 1 % at 14 years.