Interferon (IFN) is the only drug that induces viral clearance, but in patients with chronic hepatitis C, HCV-RNA clearance is achieved in only 20%-40% of patients treated with IFN for 6 months. The remaining patients have positive serum HCV-RNA, but about 5%-15% of the patients with positive serum HCV-RNA after IFN therapy showed normal alanine animotransferase (ALT) levels (incomplete response; ICR). In these patients, IFN therapy is thought to be related to the suppression of necroinflammatory reaction in the liver. The aim of this study was to evaluate the demographic, clinical, histological, and virological characteristics of patients with an ICR. In this study, ICR was defined as normalization of serum ALT, but positive HCV-RNA by reverse-transcription (RT) nested PCR at two points, 3 and 6 months after cessation of IFN therapy. By multiple logistic regression analysis, the risk ratio for ICR appearance in patients treated with IFN for more than 12 months was 3.06 compared with patients treated with IFN for less than 12 months. In patients with ICR after IFN therapy, serum ALT was often reelevated during the follow-up period. The incidence of ALT re-elevation was about 10% per year in the patients treated with IFN for less than 12 months, while the incidence of ALT reelevation in the patients treated with IFN for 12 months and more was significantly lower (P = 0.0176) by the log rank test.