BACKGROUND AIMS: We performed an open-label, multicenter, phase 3 study of the safety and efficacy of twice-daily telaprevir in treatment-naive patients with chronic hepatitis C virus (HCV) genotype 1 infection, including those with cirrhosis. METHODS: Patients were randomly assigned to groups treated with telaprevir 1125 mg twice daily or 750 mg every 8 hours plus peginterferon alfa-2a and ribavirin for 12 weeks; patients were then treated with peginterferon alfa-2a and ribavirin alone for 12 weeks if their level of HCV RNA at week 4 was /=800,000 IU/mL, 28 had fibrosis (F3-F4), 14 had cirrhosis (F4), 57 were infected with HCV genotype 1a, and 71 had the non-CC IL28B genotype. Of patients who were treated with telaprevir twice daily, 74.3 achieved SVR12 compared with 72.8 of patients who were treated with telaprevir every 8 hours (difference in response, 1.5 ; 95 confidence interval, -4.9 to 12.0 ), so telaprevir twice daily is noninferior to telaprevir every 8 hours. All subgroups of patients who were treated with telaprevir twice daily versus those who were treated every 8 hours had similar rates of SVR12. The most frequent adverse events (AEs) in the telaprevir phase were fatigue (47 ), pruritus (43 ), anemia (42 ), nausea (37 ), rash (35 ), and headache (26 ); serious AEs were reported in 9 of patients. Rates of AEs and serious AEs were similar or slightly higher among patients treated with telaprevir every 8 hours. CONCLUSIONS: Based on a phase 3 trial, telaprevir twice daily is noninferior to every 8 hours in producing SVR12, with similar levels of safety and tolerability. These results support use of telaprevir twice daily in patients with chronic HCV genotype 1 infection, including those with cirrhosis. ClinicalTrials.gov, Number: NCT01241760.