We investigated expression of HBV markers in chronic liver disease positive for antibody to HCV (anti-HCV). Sera from 107 patients with chronic non-A, non-B liver disease, 65 HBs antigen carriers with chronic liver disease and 14 asymptomatic HBV carriers were tested for the presence of anti-HCV. Anti-HCV was detected in 83 (78%) patients with chronic non-A, non-B liver disease, irrespective of the past history of blood transfusion, and anti-HCV prevalence was similar in each category of chronic liver disease. Fifty-three (64%) out of these 83 sera positive for anti-HCV has also antibodies to HBV. Anti-HBc antibody was detected frequently in liver cirrhotics with hepatocellular carcinoma than in chronic persistent hepatitis, chronic active hepatitis and cirrhotics without hepatocellular carcinoma. In addition, titers of anti-HBc antibody were significantly higher in cirrhotics with hepatocellular carcinoma than in the other groups. On the other hand, anti-HCV was detected in 7 out of 65 patients with HBV-related liver disease. Four out of these 7 were patients with HBV-related hepatocellular carcinoma. Anti-HCV was detected in none of asymptomatic HBV carriers. These findings suggest that infection with both HBV and HCV is likely to cause more serious liver disease than infection with a single agent.