Forty patients with chronic liver disease and HCC were analyzed for infection with hepatitis C (HCV) and hepatitis B (HBV) viruses. All patients were negative for HBsAg, 16 were alcoholics, 6 had previous blood transfusions and 18 had sporadic chronic hepatitis. Antibodies to HCV were determined by EIA 2nd generation. HBV-DNA was detected by PCR using primers of the precore region. Analysis of HCV-RNA was done with nested PCR amplifying the 5' non-coding region of the HCV genome, using primers complementary to nucleotides 1-20 and 305-320 and nested primers complementary to nucleotides 21-31 and 271-286 of the HCV-J1. Anti-HCV were positive in 35/40 patients (87.5%). HCV-RNA was detected by PCR in 34 patients (85%) all of them positive for the anti-HCV. HCV-RNA was detected in 70.5% of the alcohol abusers, in 100% of patients with history of transfusion(s) and 94.1% of patients with cryptogenic chronic liver disease. HBV-DNA was detected in only 2 patients. In conclusion, there is a high rate of HCV and a low rate of HBV viremia detected by PCR in Spanish patients with HCC HBsAg negative. No patient without anti-HCV presents HCV-RNA. Our results suggest that persistent HCV replication may play a role in hepatic carcinogenesis, as HBV-DNA could be found in only 5% of our HCC patients.