Enzyme-linked immunosorbent assays for detecting antibodies against hepatitis C virus and the polymerase chain reaction were tested in 82 chronic hepatitis B surface antigen carriers for their accuracy in diagnosing patients coinfected with hepatitis B and C viruses. To clarify the role of each virus in chronic hepatitis, serologic assays against hepatitis B virus were also tested. Thirteen (14.9%), 14 (17.1%) and 15 (18.3%) patients were anti-HCV positive using C100 (HCV1), JCC, and a second generation test (HCV2), respectively. HCV RNA was detected by polymerase chain reaction in 9 of 18 anti-HCV-positive cases. Although HCV1 assays were not sufficient, either the JCC or HCV2 assay detected all polymerase chain reaction-positive cases. Fifteen of 18 specimens that were positive in at least one of the three ELISA were seronegative for the hepatitis B e antigen. As judged by HBV DNA polymerase activity, titers of hepatitis B surface antigen and immunoglobulin A antibody against hepatitis B core antigen (IgA anti-HBc), activity of hepatitis B virus replication and immune response against hepatitis B virus in patients with coinfection was decreased to the level of hepatitis B virus asymptomatic carriers. These results show that hepatitis C virus appears to be the primary cause of active hepatitis in most patients with hepatitis B and hepatitis C virus coinfection.