We have reported the Keio multicenter randomized trial of interferon-alpha 2b treatment for chronic hepatitis C, hypothesizing that disappearance of serum hepatitis C virus (HCV) ribonucleic acid (RNA) during the first 2 days by one does administration of interferon is a predictive factor of final response to the high-dose interferon treatment. In this study we quantified HCV RNA by multicyclic reverse transcription-polymerase chain reaction in the stored sera of the same patients with our previous study. The multivariate analysis confirmed that the pretreatment HCV RNA levels and HCV genotype were significantly correlated with the response to the 6-month course interferon treatment. Although the relationship between decreased HCV RNA titers within the first 2 days after one dose administration of interferon and the efficacy of the therapy was not obtained, the cases in which HCV RNA disappeared within 2 days significantly responded to the 6-month course treatment (73.7% vs 12.5% in other cases, p < 0.01). The present study has confirmed the hypothesis suggested in the previous study that clearance of HCV RNA during the first 2 days has a predictive value for the final outcome.