We comprehensively studied the cellular immune response during acute human hepatitis C virus (HCV) infection by monthly prospective sampling of persons at high risk of infection. In 19 of 23 subjects, interferon-gamma-secreting T cells specific for 1 or more peptides spanning the entire HCV polyprotein were detected 1 to 3 months after infection. The median time to development of interferon gamma responses to HCV peptides was 33 days (range, 29-50 days), and these responses peaked between 180 and 360 days. Nineteen subjects had sufficient follow-up to determine outcome, with 15 (79%) developing persistent viremia and 4 (21%) clearing viremia spontaneously. Of those with progression to chronic infection and detectable T cell responses, all lost recognition of one or more antigens recognized during acute infection, and the median reduction in the magnitude of responses was 85%. Most significantly, despite ongoing viremia, those who had persistent infection did not develop new epitope specificities after the first 6 months of infection. In conclusion, in most individuals, the CD8+ T cell responses generated early in HCV infection decline in peripheral blood and are not replaced with new responses. Supplementary material for this article can be found on the HEPATOLOGY website (http://www.interscience.wiley.com/jpages/0270-9139/suppmat/index.html).