Parameters have been studied to predict responses to interferon (IFN) therapy for chronic hepatitis C, but the definition of a response, the times at which responses were assessed, and the pretreatment parameters considered differ markedly from study to study. Thus, 113 patients with chronic hepatitis C were treated 3-6 months with 3 MU of IFN-alpha 2a three times a week and assessed for pretreatment parameters predictive of responses to IFN. In a multivariate analysis, a biochemical response (normal aminotransferase activity) at the end of treatment was significantly associated with low body weight, normal gamma-glutamyl transpeptidase activity, and a pretreatment hepatitis C virus (HCV) genotype other than 1. Six months after the end of treatment, a low virus burden and a lack of anti-HCV IgM core antibodies were independently associated with sustained virologic response (i.e., normal aminotransferase activity and HCV RNA negativity). Therefore, these pretreatment parameters should be taken into account when individual treatment protocols are designed.