Background/Aims: It has been suggested that patients with chronic hepatitis B virus(HBV) infec- tion may have a superimposed form of liver disease, such as hepatitis C virus(HCV) infection. The significance and specificity of anti-HCV assays are still questionable in chronic HBV carriers. Chronic hepatitis B patients with anti-C100 are more likely to have liver cirrhosis, and reciprocal inhibition of replication in dual infections of HBV and HCV may exist. The aim of this study is to assess the prevalence of anti-C100 and the clinical relevance in chronic HBV carriers by testing the serological assays for HCV antibodies and serum HCV-RNA. Methods: Anti-C100 EIA was tested in 116 chronic HBV carriers. And anti-C100 was tested repeatedly during the follow-up period of over 6 months in anti-C100 positive cases. Anti-C100 positive sera were reassessed by the second generation anti-HCV EIA, second generation recombinant immunoblot assay(RIBA) and HCV- RNA by combined RT-PCR method and HBV-DNA by PCR using primers Pre-S region. Results: Among 116 chronic HBV carriers, anti-C100 was positive in 22 cases(18.9%). Anti-C100 positive cases were more likely to have liver cirrhosis. In 34(22 from 116 cases and additional 12 cases) anti-C100 positive cases, optical density(OD) of anti-C100 EIA was less than 2.0 in 80% of the cases and negative seroconversion was noticed in 53% during the follow-up periocL. RIBA was positive in 8 cases(23.5%) and HCV-RNA was detected in 3 cases(8.8%). HCV-RNA was not detected in 5 cases despite RIBA positive results. HBV-DNA(PCR) was not detected in the 3 HCV-RNA positive cases. Conclusions: These results suggest that serologic tests for HCV antibodies are inadequate to prove the HCV superinfection in chronic HBV carriers. Detection of HCV-RNA by using RT-PCR may be useful in confirming the prevalence of HCV superinfection and recipracal relation of HBV and HCV in chronic HBV carriers. (Korean J Gastroenterol 1996;28: 47 - 56)