The aim of the study was to investigate whether an "inapparent" coinfection by hepatitis B virus (HBV) in anti-HCV-positive chronic liver disease patients may influence interferon (IFN) response. Fourteen anti-HCV-positive, hepatitis B surface antigen (HBsAg)-negative but serum HBV-DNA-positive patients and 111 anti-HCV-positive, HBsAg-negative, and HBV-DNA-negative patients with chronic hepatitis were treated with 3 MU of recombinant alpha-2a IFN 3/week for 1.2 months. Serum HBV-DNA and HCV-RNA were determined before treatment, after 6-12 months, and at the time of alanine aminotransferase (ALT) flare-up by HBV polymerase chain reaction (PCR) and HCV PCR, respectively. IgM anti-HBc were tested using the IMx Core-M assay (Abbott Laboratories, North Chicago, IL). By the end of treatment, ALT values had become normal in 4/14 HBV-DNA-positive patients (28%), but all "responders" (4/4) relapsed. IgM anti-HBc was detected both before treatment and during ALT elevation in three patients and only during ALT relapse in another three. In the remaining 111 patients, a biochemical response to IFN treatment was observed in 54% and relapse of ALT values in 47%. "Inapparent" HBV/HCV coinfection may be implicated in cases of resistance to IFN. HBV replication and HBV-related liver damage may persist in patients in whom HCV replication was inhibited by current doses of IFN, as suggested also by the presence of IgM anti-HBc in some cases. Further studies will show the effect of different treatment schedules. HBV-DNA and/or IgM anti-HBc detection with very sensitive methods may be important both as a prognostic factor and as a tool for better understanding of intervirus relationships and mechanisms involved in multiple hepatitis virus infections.