In chimpanzees and in vitro cell culture studies, hepatitis C infection has been shown to suppress hepatitis B virus expression. In addition, hepatitis C infection can cause much more severe liver disease in patients chronically infected with hepatitis B virus. The aims of the present study were to determine the prevalence of hepatitis C infection in asymptomatic chronic hepatitis B Hong Kong Chinese patients and the histological changes and hepatic expression of hepatitis B virus and hepatitis C virus. Five hundred and seventy-one Hong Kong Chinese asymptomatic chronic hepatitis B patients were studied. Only four (0.7%) were hepatitis C virus antibody positive; they were also all positive for hepatitis C viral RNA in serum by reverse transcription-polymerase chain reaction (RT-PCR). Portal lymphoid aggregates and bile duct damage was noted in the liver sections of three of the four patients. Hepatic expression of hepatitis B surface antigen was detected in three patients; none had detectable hepatitis B core antigen. By branched DNA assay, serum hepatitis B DNA could not be detected in any of the four patients, but three had hepatitis C RNA. By in situ RT-PCR, hepatitis C RNA was detected in the cytoplasm of three of the four patients. These findings suggest that hepatitis C coinfection in asymptomatic chronic hepatitis B patients is uncommon in Hong Kong Chinese and active hepatitis B viral replication is absent in these patients.