Hepatitis B virus (HBV) and hepatitis C virus (HCV) are not only hepatotropic but possibly hematotropic. Recent studies showed the presence of HBV in lymphoma cells of extrahepatic origin. In the current study, we examined the presence of HBV DNA and HCV RNA in the tumor tissues of nine patients with primary hepatic lymphoma (PHL). Immunohistochemical study using polyclonal anti-HCV antibody was possible in four cases. The age of the patients ranged from 45 to 78 years (median, 58 yr), with a male-to-female ratio of 2:1. A history of chronic hepatitis was found in three patients and of cirrhosis in one. Histologically, all of the cases were non-Hodgkin's lymphoma of B-cell phenotype, with a diffuse large cell type being the most common. Polymerase chain reaction using both the S and X region primers failed to detect HBV DNA in the lymphoma tissues. The HCV genome was detected by in situ hybridization in the tumor cells but not in the surrounding hepatocytes in the one case of cirrhosis, which probably resulted from a blood transfusion more than 20 years previous; immunohistochemical analysis revealed positive staining for anti-HCV antibody in the cytoplasm of lymphoma cells in this case. In two cases, positive signals were found only in the hepatocytes surrounding the lymphoma. This is the first report showing the presence of the HCV genome in the lymphoma cells of PHL. HCV could be involved in development of PHL directly or via exogenic antigenic stimulus from HCV-infected hepatocytes.