This equally randomized (1:1), double-blind, parallel arm study will assess the safety and antiviral efficacy of RO5024048 added to standard Pegasys (peginterferon alfa-2a) plus Copegus (ribavirin) therapy in patients with chronic hepatitis C genotype 1 or 4. Patients in arm A will receive RO5024048 (1000mg orally twice daily) for 24 weeks in addition to Pegasys (180 micrograms sc weekly) and Copegus (1000mg or 1200mg orally daily). Patients achieving a rapid virological response (RVR) at week 4, sustained through week 22, will stop all treatment at week 24; non-RVR patients will continue treatment with Pegasys and Copegus for another 24 weeks up to week 48. Patients in arm B will receive standard treatment with Pegasys (180 micrograms sc weekly) and Copegus (1000mg or 1200mg orally daily) for 48 weeks. Anticipated time on study treatment is up to 48 weeks. Target sample size is <200.

Inclusion Criteria:

• adult patients, 18-70 years of age
• hepatitis C, genotype 1 or 4, of over 6 months duration
• treatment-naïve
• negative pregnancy test; female patients of childbearing age and male patients with female partners of childbearing age must use two forms of contraception during treatment and following the last dose of ribavirin in accordance with locally approved label for ribavirin

Exclusion Criteria:

• pregnant or breast feeding females or male partners of pregnant females
• previous interferon or ribavirin based therapy or investigational anti-HCV agent
• systemic antiviral therapy with established or perceived activity against HCV \