Hepatitis C virus (HCV) is the major etiologic agent of parenterally transmitted non-A, non-B hepatitis. To determine whether there is a relationship between this virus agent and hepatocellular carcinoma (HCC), the sera of patients with HCC and chronic hepatitis were assessed using a sensitive immunoassay for HCV antibody. Anti-HCV was detected in 65% of 132 patients with HCC, without any relationship with the presence of the hepatitis B surface antigen (HBsAg). The prevalence (74%) of anti-HCV was high, as expected in patients with putative non-A, non-B cirrhosis also. The prevalence of anti-HCV was less in patients with HBsAg-positive cirrhosis (28%) and in patients with disease not related to viral hepatitis and healthy controls (8%). These data suggest, but do not prove, that HCV is an important factor associated with HCC.