One hundred eight patients with hepatocellular carcinoma (HCC) underwent serologic testing for HBsAg, antibodies to HBs (anti-HBs), HBc (anti-HBc) and HCV (anti-HCV), and an assay for hepatitis B virus (HBV) DNA by the polymerase chain reaction (PCR). Ten (9.3%) of the 108 patients were positive for HBsAg and 96 (88.8%) were positive for anti-HCV. Both HBsAg and anti-HCV were present in 4 (3.7%) of the 108 patients. HBV DNA was detected in the serum of all patients who were positive for HBsAg and 9 (9.2%) of the 98 patients who were HBsAg-negative. Anti-HBc was positive and anti-HBs was negative in 8 of these 9 HBsAg negative patients with HBV DNA. HBsAg, HBV DNA and anti-HCV were negative in 6 (5.5%) of the 108 patients. These results suggest that HCV is highly associated with HCC and that HBV is occasionally associated with HCC even when HBsAg is negative.