We retrospectively analyzed the relationship between formation of HCV-bearing immune complexes (HCV-IC) and response to interferon (IFN) therapy for chronic hepatitis C (CH-C). Five CH-C patients with complete response (CR) to IFN therapy and pre-treatment HCV RNA titer measured by Amplicor HCV-monitor and five stage (biopsy proved fibrosis of the liver)-matiched CH-C patients who were non-esponders (NR) to IFN therapy were enrolled. Pretretment sera were separated into HCV-IC (supernatant) and anti-HCV antibody-free virions (pellet) by addition of anti-human immunoglobulins and centrifugation. HCV RNA was quantified in the supernatant and the pellet, by real-time PCR and semi-quantitative RT-PCR respectively. There were discrepancies in the HCV RNA titers measured by Amplicor and the results of real-time PCR. We found a larges quantity of HCV RNA in the supernatan and a smaller quantity in the pellet in CR than in NR by using by real-time PCR. Our data indicate that response to interferon therapy for CH-C patients may be associated with the conditions underlying the formation of HCV-IC.