SUMMARY. Chronic coinfection with the hepatitis B (HBV) and hepatitis δ (HDV) viruses is known to cause severe liver disease, but the importance of coinfection with hepatitis C virus (HCV) and HBV has not been well documented. In the present study, the clinical and pathological severity of liver disease among patients with hepatitis resulting from multiple viruses was examined and an open trial of the efficacy of interferon-α2b (IFN-α) treatment was conducted. Nineteen patients with chronic HBV and HCV infection and 17 with HBV, HCV and HDV infection were studied: 12 in each group underwent liver biopsy. For each coinfected patient, two patients infected with HCV alone were selected as controls, and these were matched for age and risk factor and were estimated to have been infected for a similar duration. Coinfection with HBV and HCV or HBV, HCV and HDV was associated with more severe liver disease than HCV alone (P < 0.01); total Scheer score, portal and lobular inflammation and fibrosis were all worse in coinfected subjects. Eight patients with chronic HBV and HCV were treated with recombinant IFN-α2b [3 million units (MU), thrice weekly for 6 months]. Liver function tests normalized in two patients and one lost hepatitis B surface antigen (HBsAg). Seven patients with hepatitis B, C and δ coinfection were treated with the same regimen and only one normalized serum alanine aminotransferase (ALT) during (and after) treatment. It is concluded that coinfection with multiple hepatitis viruses is associated with histologically more severe liver disease than HCV alone. Short-and long-term responses to doses of IFN-α that are used to treat HCV are infrequent, but further studies are required to determine whether higher-dose IFN-α may benefit patients with combined hepatitis virus infections.