When patients chronically infected with hepatitis C virus (HCV) are placed on antiviral therapy with pegylated interferon (IFN)-โฃ or IFN-โฃ plus ribavirin (RBV), HCV RNA generally declines in a biphasic manner. However, a triphasic decline has been reported in a subset of patients. A triphasic decline consists of a first phase (1-2 days) with rapid virus load decline, followed by a "shoulder phase" (4-28 days) in which virus load decays slowly or remains constant, and a third phase of renewed viral decay. We show that by including the proliferation of both uninfected and infected cells, a viral kinetic model can account for a triphasic HCV RNA decay. The model predicts that a triphasic decline occurs only in patients in which a majority of hepatocytes are infected before therapy. The shoulder phase does not represent the intrinsic death rate of infected cells, but rather the third phase slope is close to the intrinsic death rate of infected cells when overall drug efficacy is close to 1. Conclusion: Triphasic responses can be predicted from a generalization of existent viral kinetic models through the inclusion of homeostatic proliferation of hepatocytes. This generalized model can also explain the viral kinetics seen in flat partial responders. Finally, the enhanced third phase in patients treated with IFN-โฃ in combination with RBV versus patients treated with IFN-โฃ alone can be explained by a mutagenic effect of RBV against HCV. (HEPATOLOGY 2007;46:16-21.