The possible role of hepatitis viruses in hepatocarcinogenesis was summarized. Some 2769 of 3017 patients (92%) with hepatocellular carcinoma had advanced liver diseases such as chronic hepatitis or liver cirrhosis in Japan; and 129 of 135 patients (95%) tested positive for either hepatitis B surface antigen or hepatitis C virus antibodies. Therefore, most patients with hepatocellular carcinoma had chronic viral hepatitis in Japan. When 212 patients with chronic hepatitis and 115 with liver cirrhosis were followed (average 8.5 and 6 years, respectively), 23 (10.8%) and 43 (37.4%) of them were complicated with hepatocellular carcinoma, respectively. The ratio of these developing hepatocellular carcinoma was significantly higher among patients with viral infections than in those without them. Although these findings demonstrate a close relationship of hepatitis viral infections with hepatocarcinogenesis, the direct role of the virus on carcinogenesis has not been elucidated. The following issues were discussed with regard to hepatitis B virus: (1) "insertion-mutagenesis," (2) transactivation of X protein, and (3) genomic instability by viral integration. Since hepatocellular carcinoma was complicated after a long period of chronic viral hepatitis, further analyses about not only virology but changes of host chromosomal DNA during hepatitis are necessary for understanding its role.