Chronic infection with hepatitis C virus (HCV) is a major global health problem. One way HCV may evade the host immune response is by inhibiting the production of type I interferon (IFN). In addition, the standard treatment for chronic HCV infection involves treatment with IFN- (or its pegylated derivative), alone or in combination with ribavirin. Therefore, it is believed that an important reason that most HCV-infected individuals progress from acute to chronic infection is due to a defect in the host response. In this study, we examined the host response to HCV infection in a cohort of patients enrolled in the UTHSC Cooperative HCV Research Center by determining levels of biologically active IFN in the sera of patients. We found that 15 of 35 enrolled HCV-infected patients show serum levels of IFN (ranging from 2 to 40 IU/mL) before initiation of therapy. Uninfected individuals do not have circulating levels of IFN. Basal IFN levels do not correlate with the clinical response to therapy, nor do they reflect the age, sex, or race of patients. These results suggest that the differential response of patients most likely reflects a defect in the later stages of the host innate immune response, such as the cellular response to endogenous or exogenous IFN. In contrast, the early stage of the host immune response in vivo of many HCV-infected patients (∼40%) is intact as determined by IFN production.