We investigated autoimmunity, as assessed by hypergammaglobulinemia and the presence of autoantibodies including anti-nuclear antibodies (ANA) and anti-liver membrane antibodies (LMA), in 149 patients with chronic hepatitis C, 55 patients with chronic hepatitis B and 11 patients with autoimmune hepatitis. There was no significant difference in the incidence of these autoantibodies between chronic hepatitis C and chronic hepatitis B. Nine patients with chronic hepatitis C satisfied the serological criteria of autoimmune hepatitis (ANA positive and gammaglobulin or serum IgG greater than 2500 mg/dl), but none of the patients with chronic hepatitis B met the criteria. This suggests that autoimmunity is greater in chronic hepatitis C than in chronic hepatitis B. Of the 9 patients with chronic hepatitis C, all 4 patients tested for human leukocyte antigen (HLA) phenotype had HLA-DR4, which is known to be associated with autoimmune hepatitis in Japanese patients. We believe that hepatitis C virus (HCV) infection enhances the initiation and perpetuation of autoimmunity in susceptible individuals.