Because hepatitis B virus (HBV) and hepatitis C virus (HCV) have the same transmission routes, dual infection may occur and even persist in the same patient. The reported series on seroprevalence of HCV indicate that HCV is found in more than 10% of HBV-infected patients worldwide. HCV superinfection in patients with chronic HBV infection tends to cause severe and progressive liver disease that is resistant to interferon therapy. Paradoxically, HCV exerts a suppressive effect on HBV and may enhance seroclearance of HBV antigens, or even usurp the role of HBV as the agent for continuing hepatitis. In view of the complex dynamism of viral interaction, the importance of HCV assay and the necessity of monitoring patients with chronic HBV infection in clinical studies cannot be overemphasized. The basic mechanisms that regulate the viral interactions largely remain to be investigated.