BACKGROUND: In patients with chronic infection with hepatitis C virus (HCV) genotype 1, treatment with peginterferon alfa and ribavirin for 48 weeks results in rates of sustained virologic response of 40 to 50%. Telaprevir is a specific inhibitor of the HCV serine protease and could be of value in HCV treatment. METHODS: A total of 334 patients who had chronic infection with HCV genotype 1 and had not been treated previously were randomly assigned to receive one of four treatments involving various combinations of telaprevir (1250 mg on day 1, then 750 mg every 8 hours), peginterferon alfa-2a (180 microg weekly), and ribavirin (dose according to body weight). The T12PR24 group (81 patients) received telaprevir, peginterferon alfa-2a, and ribavirin for 12 weeks, followed by peginterferon alfa-2a and ribavirin for 12 more weeks. The T12PR12 group (82 patients) received telaprevir, peginterferon alfa-2a, and ribavirin for 12 weeks. The T12P12 group (78 patients) received telaprevir and peginterferon alfa-2a without ribavirin for 12 weeks. The PR48 (control) group (82 patients) received peginterferon alfa-2a and ribavirin for 48 weeks. The primary end point, a sustained virologic response (an undetectable HCV RNA level 24 weeks after the end of therapy), was compared between the control group and the combined T12P12 and T12PR12 groups. RESULTS: The rate of sustained virologic response for the T12PR12 and T12P12 groups combined was 48% (77 of 160 patients), as compared with 46% (38 of 82) in the PR48 (control) group (P=0.89). The rate was 60% (49 of 82 patients) in the T12PR12 group (P=0.12 for the comparison with the PR48 group), as compared with 36% (28 of 78 patients) in the T12P12 group (P=0.003; P=0.20 for the comparison with the PR48 group). The rate was significantly higher in the T12PR24 group (69% [56 of 81 patients]) than in the PR48 group (P=0.004). The adverse events with increased frequency in the telaprevir-based groups were pruritus, rash, and anemia. CONCLUSIONS: In this phase 2 study of patients infected with HCV genotype 1 who had not been treated previously, one of the three telaprevir groups had a significantly higher rate of sustained virologic response than that with standard therapy. Response rates were lowest with the regimen that did not include ribavirin. (ClinicalTrials.gov number, NCT00372385.)