BACKGROUND/AIMS: Cross-sectional studies have documented that 2-10% of patients who are chronically infected with hepatitis C virus (HCV) are also positive for hepatitis B virus (HBV) surface antigen (HBsAg). Data related to HCV-HBV coinfection are lacking in Korea. This study evaluated the clinical characteristics, the treatment efficacy of peginterferon alfa plus ribavirin, and the changes induced by such treatment in HBV status in chronic hepatitis C (CHC) patients coinfected with HBV. METHODS: Eighteen (2.37%) HBsAg-positive CHC patients were selected from among the 758 subjects from the K(G)yeonggi-Incheon Peginterferon alfa and ribavirin in chronic hepatitis C Treatment (KIPECT) study, which evaluated the treatment efficacy and safety of peginterferon alfa plus ribavirin in CHC patients. Data on changes in the status of HBV infections were obtained. RESULTS: HCV genotype 1b was the most common (44%). The overall sustained virologic response rate was 72% in all patients, and 60% and 87.5% in genotypes 1 and 2, respectively. Two of the 18 patients were positive for HBeAg, and 15 had baseline HBV DNA level of less than 2,000 IU/mL. Two of the three whose levels exceeded this threshold showed no detectable DNA after treatment. After the completion of treatment, serum HBV DNA levels were increased in the two patients whose baseline HBV DNA levels were less than 2,000 IU/mL. CONCLUSIONS: The prevalence of HBV coinfection in CHC patients was 2.37% and most of the patients were inactive carriers. The treatment efficacy was similar to that of HCV mono-infection. Reactivation of HBV replication was observed in some patients after CHC treatment.