Although CD8⁺ T cells initially suppress human immunodeficiency virus (HIV) replication, cytotoxic T-cell precursor frequencies eventually decline and fail to prevent disease progression. In a longitudinal study including 16 individuals infected with HIV-1, we studied both the number and function of HIV-specific CD8⁺ T cells by comparing HLA-peptide tetramer staining and peptide-induced interferon-γ (IFN-γ) production. Numbers of IFN-γ–producing T cells declined during progression to acquired immunodeficiency syndrome (AIDS), whereas the number of tetramer⁺ T cells in many individuals persisted at high frequencies. Loss of IFN-γ–producing T cells correlated with declining CD4⁺ T-cell counts, consistent with the need of CD4⁺ T-cell help in maintaining adequate CD8⁺T-cell function. These data indicate that the loss of HIV-specific CD8⁺ T-cell activity is not due to physical depletion, but is mainly due to progressively impaired function of HIV-specific CD8⁺ T cells.