Thirty-three patients fulfilling stringent criteria for autoimmune chronic active hepatitis (CAH) were tested for antibodies against hepatitis C virus (anti-HCV) with an enzyme immunoassay. Eleven of the 33 (33%) patients were anti-HCV-reactive in a serum drawn before initiation of immunosuppressive therapy. Anti-HCV-reactive patients had significantly higher median s-IgG level (32.3 g/l) than nonreactive patients (23.5 g/l) (p less than 0.01). When in remission with normalized s-IgG levels as a result of the immunosuppressive therapy, only 1 of the 11 initially reactive patients was still reactive. Sera from patients with acute autoimmune CAH and high s-IgG levels may be nonspecifically reactive for anti-HCV.