The present study was designed to determine the frequency of hepatitis B virus (HBV) and hepatitis C virus (HCV) infection in patients with hepatocellular carcinoma (HCC) in Nagasaki Prefecture, Japan. We examined the clinical features of 1019 patients with HCC who visited our hospitals between January and December 1999. The ratio of men to women was 709 : 310, and the peak incidence of HCC was in the seventh decade of life in both men and women. In the majority of the patients, HCC showed association with HCV infection (74%) compared with HBV infection (17%). HBV-associated HCC was more common in young patients, while HCV-associated HCC was more common in patients with a history as a "daily drinker", or with a history of blood transfusion, liver cirrhosis, and persistently high serum transaminases before the diagnosis of HCC. HCC was initially suspected by ultrasonography or computed tomography in 776 of the 874 patients for whom there was a history of mode of detection of HCC (89%). Tumor size at the time of diagnosis of HCC in patients who had been regularly followed up for liver diseases at our hospitals was significantly smaller than that in patients who were not followed up regularly before the diagnosis ( P < 0.01). Our results indicate that the proportions of patients with HBV or with HCV infection among HCC patients in Nagasaki Prefecture are similar to those found in a nationwide survey in Japan, and there are some differences between the clinical manifestations of HBV- and HCV-associated HCC. Our results emphasize the importance of close follow-up for the high-risk group (i.e. those with HBV- or HCV-associated chronic liver diseases) for the early detection of HCC.