Concurrent infections with HGV and/or HCV (HGV/HCV) were investigated in 196 patients with HBV-related chronic liver disease (115 chronic hepatitis, 31 liver cirrhosis, 50 hepatocellular carcinoma), and in 100 HBsAg carriers. Coinfections were detected in 18 (9.2%) patients with HGV (10) or HCV (5) or both agents (3), but in none of the HBsAg carriers. Patients with coinfection were more frequently exposed to blood transfusions (55.6% vs 5.6%) and also were more commonly anti-HBe positive. Serum levels of HBV-DNA were lower in patients with HCV coinfection than in those coinfected with HGV. Interferon was administered to 39 patients with chronic active hepatitis including 7 patients with HGV/HCV coinfection. Sustained clearance of HBV-DNA was observed in 10 (25.6%) patients who were solely infected with HBV. These patients were significantly younger and had much lower histological scores than non-responders. Patients with HCV coinfection had significantly higher pre-treatment histological scores than those without HCV. After interferon treatment, a significant reduction in histological scores was observed in all patients except those coinfected with HGV/HCV. None of the 7 patients with coinfection had sustained clearance of HBV-DNA or HCV-RNA, and only one had cleared HGV-RNA. These results suggest that parenteral exposure is a risk factor for HGV/HCV coinfection in chronic HBV infection. HGV infection shows no significant impact on chronic HBV infection. HCV coinfection appears to inhibit HBV replication, but causes more severe chronic hepatitis and increases resistance to interferon therapy.