Increasing numbers of patients with hepatocellular carcinoma (HCC) have been reported in Japan. In this paper, we investigated the role of hepatitis C virus (HCV) in HCC and the reason for the increase, using patients admitted to our university hospital from 1945 to 1992. 99 (73%) of 135 patients with HCC were positive for anti-HCV. Prospective studies demonstrated that 22 of 158 (14%) patients with chronic hepatitis C, and 31 of 70 (44%) cirrhotic patients with anti-HCV developed to HCC during the follow-up period (10.1 +/- 3.3 and 7.3 +/- 3.5 years, respectively). Prolonged survival of cirrhotic patients during past decades would also contribute to the increasing number of HCC cases as well as the number of HCV infections in Japan.