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To assess the contribution of the recently identified hepatitis C virus to chronic liver diseases of unknown cause and chronic hepatitis attributed by exclusion to non-A, non-B hepatitis, we tested for antibody to hepatitis C in hepatitis B surface antigen-negative patients with a spectrum of chronic liver diseases. Antibody to hepatitis C virus, a marker of hepatitis C infection, was detected with a first-generation radioimmunoassay at the following frequencies in the following patient groups: 69% of transfusion-associated non-A, non-B hepatitis; 53% of non-transfusion-associated non-A, non-B hepatitis; 26% of hepatitis B surface antigen-negative hepatocellular carcinoma; 8% of cryptogenic cirrhosis; 5% to 7% of autoimmune chronic liver diseases; 19% of patients with miscellaneous types of chronic liver disease; and 0.67% of healthy controls. Among non-transfusion-associated cases, 81% with a history of intravenous drug use but only 18% with occupational exposure as health workers had antibody to hepatitis C virus. Among cases of hepatocellular carcinoma, 63% of Japanese patients but only 11% of American patients had evidence of hepatitis C infection. Comparison in a subgroup of 79 serum samples of a second-generation radioimmunoassay with the first-generation assay demonstrated a 12% increase in antibody frequency from 30% to 42%. We conclude that hepatitis C plays a substantial role in transfusion-associated and non-transfusion-associated non-A, non-B hepatitis as well as in hepatocellular carcinoma, especially in Japan, a limited role in cryptogenic cirrhosis, and essentially no role in autoimmune chronic liver diseases. Application of more sensitive immunoassays will increase the frequency of antibody seropositivity in all subgroups, but relative distinctions among risk groups are likely to remain.