Spontaneous recovery occurs in a minority of patients with acute hepatitis C but is associated with vigorous and long-lasting cellular immune responses. Treatment-induced recovery can be achieved in the majority of patients who are treated in the acute phase, but the kinetics and mechanisms of viral clearance and immune responsiveness are not known. Both direct antiviral effects and indirect immune-mediated effects, such as immune modulation of Th2 to Th1 responses and prevention of exhaustion of cellular responses by rapid reduction of viral titer, have been proposed. To investigate how early antiviral therapy affects hepatitis C virus (HCV)-specific T cell responses, we performed detailed prospective clinical, virological, and immunological studies on 7 patients with acute hepatitis C who received antiviral therapy and were followed at 2 to 4 week intervals for 1 to 2 years. The total CD4(+) and CD8(+) cell response was analyzed with 600 overlapping HCV peptides and 6 proteins by ex vivo enzyme-linked immunospot (ELISpot), intracellular cytokine staining, and proliferation assays. In contrast to earlier studies with selected HCV epitopes, this extended analysis detected multispecific interferon gamma(+) (IFN-gamma(+)) responses in each patient, even in the absence of T-cell proliferation. After initiation of antiviral therapy (at a mean of 20 weeks after infection), all sustained responders demonstrated gradually decreasing, then nearly absent HCV-specific T-cell responses, whereas the sole patient who developed viral breakthrough after initial HCV control maintained cellular immune responses. In conclusion, a sustained response to antiviral therapy was not associated with a lasting enhancement of HCV-specific T-cell responsiveness in the blood.