Virus-specific CD8⁺ T cells are associated with declining viremia in acute human immunodeficiency virus (HIV)1 infection, but do not correlate with control of viremia in chronic infection, suggesting a progressive functional defect not measured by interferon γ assays presently used. Here, we demonstrate that HIV-1–specific CD8⁺ T cells proliferate rapidly upon encounter with cognate antigen in acute infection, but lose this capacity with ongoing viral replication. This functional defect can be induced in vitro by depletion of CD4⁺ T cells or addition of interleukin 2–neutralizing antibodies, and can be corrected in chronic infection in vitro by addition of autologous CD4⁺ T cells isolated during acute infection and in vivo by vaccine-mediated induction of HIV-1–specific CD4⁺ T helper cell responses. These data demonstrate a loss of HIV-1–specific CD8⁺ T cell function that not only correlates with progressive infection, but also can be restored in chronic infection by augmentation of HIV-1–specific T helper cell function. This identification of a reversible defect in cell-mediated immunity in chronic HIV-1 infection has important implications for immunotherapeutic interventions.