To determine the frequency and significance of antibody to hepatitis C virus (anti-HCV) in severe autoimmune chronic active hepatitis, we tested sera from 85 cortico-steroid-treated patients by an enzyme immunoassay. Seropositive patients were assessed for specific antibodies to hepatitis C virus-encoded antigens by recombinant immunoblot assay. The findings in patients with and without anti-HCV were contrasted, and the frequency of seropositivity was compared with that in patients who had other types of chronic liver disease and in normal adults. Only 5 of the 85 patients with autoimmune hepatitis (6%) were seropositive for anti-HCV, and only 2 of these patients were reactive by recombinant immunoblot assay. The frequency of seropositivity in autoimmune hepatitis was not significantly different from that in hepatitis B surface antigen-positive (9%) and cryptogenic (18%) disease, but it was significantly less than that in posttransfusion chronic active hepatitis (6% versus 75%; P less than 0.001). Two patients became seronegative after corticosteroid therapy; both had been nonreactive by recombinant immunoblot assay. Four of the seropositive patients entered remission during corticosteroid therapy, including three whose sera were nonreactive to virus-encoded antigens. We conclude that anti-HCV occurs infrequently in corticosteroid-treated severe autoimmune hepatitis and that antibodies detected by enzyme immunoassay may be nonreactive to hepatitis C virus-encoded antigens. Seropositive patients who are nonreactive by immunoblot assay may still respond to corticosteroid therapy and become seronegative during treatment.