The development of hepatocellular carcinoma (HCC) is very closely associated with chronic liver disease. In the present study, the prevalence of the hepatitis B virus (HBV) and hepatitis C virus (HCV) infection as a causative role in the development of HCC was analysed in 253 patients with HCC, who were admitted to our hospital during 1976-90. Among these patients, 68 (27%) were positive for HBsAg but negative for anti-HCV antibody (group I); in contrast, 147 (58%) were negative for HBsAg but positive for anti-HCV antibody (group II), 19 (7.5%) were both positive (group III), and 19 (7.5%) were both negative (group IV). To evaluate the serial changes in the prevalence of HBsAg and anti-HCV antibody, changes in the number of patients were compared between group I and group II. The number of group I patients reached a peak during 1982-84 and was thereafter followed by a decreasing trend, whereas the number of group II patients steadily increased and reached a plateau over 6 recent years. These results suggest that HCV infection recently seems to play a more important role in the development of HCC than chronic HBV infection, even in the Nagasaki Prefecture, where the HBV carrier rate is higher than elsewhere in Japan.