The aim of this large-scale analysis was to assess the effect of 48-week pegylated interferon (PEG-IFN) α-2b and ribavirin (RBV) therapy on virological relapse by patients infected with hepatitis C virus (HCV) genotype 1. The relationship between virological relapse and the dose of PEG-IFNα-2b and RBV was investigated in 619 patients who had once cleared HCV RNA during PEG-IFNα-2b and RBV treatment for 48 weeks. The overall virological relapse rate was 34.1% (211 of 619). The relapse rate was 59.5% (22 of 37) for patients who received <6 mg/kg/day of RBV, even if a sufficient dose of PEG-IFNα-2b (≥1.5 μg/kg/day) was received. In contrast, the relapse rate was 28.1% (16 of 57) for patients who received ≥12 mg/kg/day of RBV, irrespective of the PEG-IFNα-2b dose. The relapse rates were significantly increased with the reduction of the RBV dose for both PEG-IFNα-2b doses of ≥1.2 and <1.2 μg/kg/week (P < 0.0001 and P = 0.0006, respectively). Moreover, the relapse rate was 41.2% (35 of 85) for patients with an early virological response (EVR) who received <6 mg/kg/day of RBV. The relapse rates were significantly increased with the reduction of the RBV dose in both those patients with an EVR and those with a late virological response (P = 0.0006 and P = 0.0088, respectively). To summarize, for HCV genotype 1 patients treated with PEG-IFNα-2b and RBV, the virological relapse of HCV was RBV dose-dependent, irrespective of the dose of PEG-IFNα or the effect of early viral kinetics.