In Japan, hepatocellular carcinoma (HCC) is one of the most prevalent cancers, with a reported fatality rate showing a consistent and significant increase in the last decade. At most, only 25% of HCC cases are positive for the hepatitis B surface antigen (HBsAg). To investigate a potential role for hepatitis C virus (HCV) in the development of HCC, sera from 105 HBsAg-negative HCC patients were collected from five districts of Japan and assayed for antibody to HCV antigen (HCVAb). A large number of these patients (76.2%) were found to be positive for the HCVAb in comparison with the reported prevalence in sera from blood donors (1.1%). A history of blood transfusion was found in 39.6% of the cases positive for HCVAb, which was significantly different to the lower rate (4.7%) observed in HCC patients who were both positive for HBsAg and negative for HCVAb (P less than 0.001).