Interferon-α (IFN-α) has been used to treat hepatitis C virus (HCV)-induced hepatitis, but it has been effective in only about half of the treated patients, with recurrence appearing in the other half. As a consequence of the possible complications associated with IFN-α and the high cost of treatment, it has become extremely important to select the proper patients for IFN-α treatment. In our previous study, we found that the quasispecies in the hypervariable region (HVR) 1 of HCV were various and that a new quasispecies can appear in non-responders and/or lead to deterioration in the patients' condition. The preliminary data we obtained in the process of our previous research led us to believe that the quasispecies of HVR 1 has something to do with the effect of IFN-α. Thus, in this investigation, we tried to determine the predictive factors of IFN-α therapy. Thirty patients with HCV infection were treated with IFN-α. Among them, 15 patients recovered after six months IFN-α treatment, but the remaining 15 patients showed no response after six months IFN-α treatment. We cloned HVR 1 DNA by reverse transcription-polymerase chain reaction (RT-PCR) and examined the quasispecies of HVR 1. As the quasispecies of HVR 1 in non-responders varied more than in the complete remission group, we concluded that the sequence variation in HVR 1 of HCV can be used to predict the effect of IFN-α.