A dynamic equilibrium between viral production and clearance characterizes untreated chronic hepatitis C viral infection. After initiating antiviral treatment, a typical multiphasic decay of viremia can be observed and analyzed using mathematical models. To elucidate the antiviral mechanism of ribavirin when used in combination with (pegylated) interferon alfa, we investigated kinetic parameters in patients with chronic hepatitis C treated with either peginterferon alpha-2a with or without ribavirin and standard interferon alpha-2b plus ribavirin for 48 weeks. Serum HCV RNA was measured frequently before, during, and at the end-of-treatment and the follow-up period. By using an appropriate model for viral dynamics, kinetic parameters were derived from nonlinear, least square fitting of serum HCV RNA quantifications. The first phase of viral decay (day 1) and the second phase of viral decay (days 2 to 21) were similar for all treatment groups. After about 7 to 28 days, a third phase of viral decay was seen in several patients, and this phase of decay was significantly faster in patients treated with peginterferon alpha-2a plus ribavirin compared with those treated with peginterferon alpha-2a alone. The decay of this third phase was associated with the virologic end-of-treatment response and sustained virologic response. In conclusion, the third-phase decay of initial viral kinetics, which may represent a treatment-enhanced degradation of infected cells, was more pronounced in patients treated with peginterferon alpha-2a plus ribavirin. This finding suggests that combination treatment leads to a better restoration of the patient's immune response.