Human hepatitis B virus (HBV) and hepatitis C virus (HCV) are two major etiologic agents of chronic hepatitis, which is closely related to the development of hepatocellular carcinoma (HCC). We investigated the role of HBV coinfection in ongoing HCV-related liver diseases in HCV-infected patients. We found a high prevalence of anti-HBc in anti-HCV-positive/HBsAg-negative HCC patients and also found a close correlation between anti-HBc positivity and integration of HBV DNA into HCC DNA of anti-HCV-positive/HBsAg-negative patients. The present data suggest that integrated HBV DNA may play an important role in the development of HCC in the anti-HCV-positive/HBsAg-negative patients carrying the anti-HBc antibody.