Combination therapy with interferon alfa (IFN-α) plus ribavirin for 24 to 48 weeks produces sustained virologic response (SVR) rate in approximately 31-47% of treatment naïve patients with chronic hepatitis C.(1-5) Patients with genotype 1 virus infection are less likely to have SVR that those with other genotypes infection, and therefore, patients infected with hepatitis C virus (HCV) genotype 1 should receive treatment for 48 weeks.(6) Recently, combination therapy with pegylated interferon alfa (pegylated IFN-α) plus ribavirin produces higher SVR rates (54-56%) than that with IFN-α plus ribavirin.(7,8) Furthermore, a large trial assessing the effect and duration of pegylated IFN-α plus ribavirin showed that the overall SVR rate was 63%. Among patients with genotype 1 HCV infection, standard dose ribavirin (1000 to 1200 mg per day) and 48 weeks of treatment were significantly more effective than low dose ribavirin (800 mg per day) or 24 weeks of treatment.(9) The SVR rate was 51% for genotype 1 patients receiving pegylated IFN-α plus standard dose ribavirin for 48 weeks, whereas only 29% and 41% for those receiving pegylated IFN-α plus low dose ribavirin and standard dose ribavirin for 24 weeks, respectively. Based on these lines of evidence, 48 weeks of therapy with pegylated IFN-α (pegylated IFN-α 2a 180 μg or pegylated IFN-α 2b 1.5 μg per kilogram body weight weekly) plus ribavirin (1000 to 1200 mg per day) is recommended to treat patients with HCV genotype 1 infection.(10) In Taiwan, a multicenter study showed that a 6 month course treatment with pegylated IFN-α plus standard dose ribavirin had a comparable SVR rate to that with IFN-α plus standard dose ribavirin (67.1% versus 63.6%) in patients with chronic hepatitis C. Subgroup analysis showed that treatment with pegylated IFN-α plus standard dose ribavirin had a significantly higher SVR rate to that with IFN-α plus standard dose ribavirin (65.8% versus 41.0%) in patients with genotype 1 HCV infection.(11) Recently, a pilot study comparing 24 and 48 weeks of pegylated IFN-α plus standard dose ribavirin in patients with genotype 1 HCV infection showed that 48 weeks of treatment is more efficacious that 24 weeks of treatment (SVR rate: 80.0% versus 48.9%).(12) However, much difference of SVR rates occurred in these two studies, making optimal therapy in Taiwanese patients infected with genotype 1 HCV difficult to be determined. In the study, we aim to investigate in a large cohort whether 48 weeks treatment with pegylated IFN-α plus standard dose ribavirin is more efficacious than 24 weeks treatment in patients with genotype 1 HCV infection.