A novel virus (GBV-C/HGV) may be associated with some liver diseases including fulminant hepatitis and acute and chronic hepatitis. On the other hand, many investigations showed that this infection does not contribute to liver disease. GBV-C/HGV has been found to occur in association with infection with other hepatitis viruses. We investigated the effect of GBV-C/HGV infection on the clinical features and interferon treatment in patients with chronic hepatitis C. A total of 262 hepatitis C virus (HCV) RNA positive patients with chronic hepatitis were examined in this study. The detection of serum GBV-C/HGV RNA was done by RT-PCR using specific primers from the NS5 regions. Interferon-alpha was given at a dose of 6 MU/day for 16 or 24 weeks. A responder was defined as a patient with ALT normalization and HCV RNA disappearance after treatment. GBV-C/HGV RNA was detected in 28 (11%) patients. No significant difference was detected in clinical features (age, sex, liver-related biochemical tests, and histological examination) between the 28 GBV-C/HGV-positive patients and the GBV-C/HGV-negative patients. Using interferon therapy for hepatitis C, the responder rates of GBV-C/HGV-positive and -negative patients were 14% and 20%, respectively. Of the 28 patients with GBV-C/HGV RNA, GBV-C/HGV RNA was tested after interferon therapy in 16 and of these GBV-C/HGV RNA was not detected in nine patients after therapy. These findings suggest that GBV-C/HGV infection dose not affect the clinical features in patients with HCV and the efficacy of interferon therapy for chronic hepatitis C.