BACKGROUND Patients with dual hepatitis B virus (HBV) and hepatitis C virus (HCV) infection have responded poorly to interferon (IFN) monotherapy. The purpose of the present paper was to assess the effect of combined IFN-alpha and ribavirin therapy in patients infected with both hepatitis B and C. METHODS Thirty-six patients received 3 or 5 MU IFN-alpha-2b thrice weekly and oral ribavirin (800-1200 mg/day) for 24 weeks. All patients had positive hepatitis B surface antigen, antibody to HCV, and HCV-RNA. Before treatment, one patient had positive hepatitis B e antigen. Eighteen patients had positive HBV-DNA tested by Amplicor (Cobas Amplicor Monitor, Roche Diagnostics, Branchburg, NJ, USA), with a mean HBV-DNA level of 3.1 +/- 0.9 log copies/mL. Another 72 patients with HCV infection alone served as controls. RESULTS Adverse events led to withdrawal in three patients receiving 5 MU IFN. Based on an intent-to-treat analysis, the biochemical response and serum HCV clearance rate at the end of 48 weeks follow up was similar in patients with dual infection and HCV infection alone (56% vs 72%; and 69% vs 71%, respectively). There was no significant difference in sustained HCV clearance rate between the 3-MU group (n = 13) and the 5-MU group (n = 23; 85% vs 61%). At the end of 48 weeks follow up, two (11%) of 18 pretreatment viremic patients had negative serum HBV-DNA (<200 copies/mL), while eight of those without pretreatment viremia had re-occurrence of HBV-DNA. CONCLUSIONS Combination therapy with IFN-alpha and ribavirin was effective in achieving sustained HCV clearance in patients with dual HBV and HCV infection, comparable to those with hepatitis C infection alone. Combination therapy using 3 MU IFN-alpha seemed as effective as 5 MU, and was well tolerated in the study population. However, large-scale control trials are necessary to clarify these findings.