Hepatocellular carcinoma (HCC) develops several years after the eradication of hepatitis C virus (HCV) by interferon therapy. Risk factors for the development of HCC are only partly understood. To elucidate the role of occult hepatitis B virus (HBV) infection in hepatocarcinogenesis in patients with sustained virologic response, the prevalences of HBV-related makers were examined. Study group comprised 16 patients with sustained virologic response (group A) and 50 with HCV (group B). Anti-HBc and anti-HBs in serum were examined by enzyme-linked immunoassay. HBV DNA in liver was examined by nested polymerase chain reaction, using primers specific for genes encoding for HBx, HBsAg, HBcAg, and HBV cccDNA. Sequence of the amplified HBV DNA for 'a' determinant of HBsAg was determined in HCC. Anti-HBc was positive in 10 of 16 in group A and 25 of 50 in group B. HBV DNA in liver was detected in 12 of 16 in group A and 21 of 50 in group B (P = 0.044). In group A, HBV DNA in liver was detected frequently in patients without cirrhosis and in those with a longer period from the time of HCV eradication to the development of HCC. Mutation in 'a' determinant of HBsAg was found in three HCC of group A. Occult HBV infection may be one of the most important risk factors in hepatocarcinogenesis of Japanese patients with sustained virologic response.