Anti-c100-3 (Ortho) was determined in the sera of 152 patients with HBs antigen-positive chronic liver diseases to assess coinfection of hepatitis B virus (HBV) and hepatitis C virus (HCV). Eleven patients (7.2%) were positive for anti-c100-3. Anti-CP-9 (Okamoto) and HCV-RNA (RT-PCR) were also examined in these 11 patients. Anti-CP-9 was detected in 7 patients and HCV-RNA was detected in all 11 patients. Four of the 11 anti-c100-3-positive patients were positive for HBe antigen (HBeAg) and others were negative. In 8 of the 11 patients, HCV was suspected to be superinfected by blood transfusion. In HBeAg-positive patients, serum glutamic pyruvic transaminase (SGPT) was elevated in relation to active replication of HBV shown by DNA-polymerase activity. The histological findings showed chronic active hepatitis, with or without cirrhosis. On the other hand, in HBeAg-negative patients, SGPT fluctuated without evidence of active replication of HBV. Active inflammation in the liver was observed in 3 of 5 HBeAg-negative patients by liver biopsy. These findings suggest that HBV might play an important role in chronic active inflammation in HBeAg-positive patients coinfected with HCV, and that HCV might be responsible for continuous inflammation in HBeAg-negative patients coinfected with HCV.