Background: There is a limited amount of published data on the interference of hepatitis B virus (HBV) on hepatitis C virus (HCV). The aim of this study was to investigate the effect of concurrent HBV infection on serum titers of HCV RNA and HCV antibody profiles in chronic HCV infection. Methods: The clinical and virological profiles (serum titers of HCV RNA, HCV genotypes and antibody profiles) of 25 patients with chronic HBV‐HCV coinfection were compared with those of 25 age‐ and sex‐matched patients with HCV infection alone. Results: Among the 25 patients with HBV‐HCV coinfection, only 3 were found hepatitis Be antigen (HBeAg) and HBV DNA positive by hybridization assays, and the other 11 were found HBV DNA positive by polymerase chain reaction. Genotype 1b was dominant in both HBV‐HCV coinfection and HCV infection alone (64% versus 84%, P > 0.1). Patients with HBV‐HCV coinfection had significantly lower alanine aminotransferase (ALAT) levels and inflammatory scores but higher fibrosis scores than those with HCV infection alone. Serum titers of HCV RNA were significantly lower in HBV‐HCV coinfection than in HCV infection alone. The frequency and relative intensity of antibody response to core, E2/NS1, NS3, and NS5 showed no significant difference between the two groups, but antibody response to NS4 was diminished significantly in HBV‐HCV coinfection. Conclusions: In HBV‐HCV coinfection, serum levels of HBV DNA are usually low or undetectable. Concurrent HBV infection, however, could interfere with HCV replication and suppress antibody response to NS4. The biological significance of selective inhibition of humoral immune response to NS4 in HBV‐HCV coinfection should be further studied.