Abstract: Background: To assess the efficacy of 24- or 48-week peginterferon/ribavirin treatment of Taiwanese patients with chronic hepatitis C virus genotype-1b (HCV-1b) infection, and to identify subgroups of patients in whom the 48-week treatment has benefits. Methods: We assigned 60 patients receiving peginterferon-α-2b (80–100 mcg/week) plus ribavirin (1000–1200 mg/day), depending on body weight, for 24 or 48 weeks, with a 3:1 randomization ratio. Results: The sustained virological response (SVR) rate was significantly higher in the 48-week (80.0%, 12/15) than in the 24-week group (48.9%, 22/45, P<0.05). The 60 patients were classified into two subgroups according to the presence of unfavorable baseline predictors: viral loads ≥400 000 IU/ml or a hepatic fibrosis score of 3–4. In 19 patients without an unfavorable predictor, the SVR rate was comparable in the 24-week (78.6%) and 48-week (75.0%) groups; in patients with either unfavorable predictors, the SVR rate was significantly higher in the 48-week (81.1%, 9/11) than in the 24-week group (36.7%, 11/30, P=0.015). The discontinuation rate was significantly higher in the 48-week (20.0%, 3/15) than in the 24-week group (2.2%, 1/45, P<0.05). Conclusion: A 48-week course of peginterferon-α-2b/ribavirin was more effective than a 24-week course in Taiwanese HCV-1b patients, mainly in those with high viral loads and/or advanced hepatic fibrosis.