Human T-lymphotropic virus type 1 (HTLV-1)–associated myelopathy/tropical spastic paraparesis (HAM/TSP) is an inflammatory neurologic disease caused by HTLV-1 infection, in which HTLV-1–infected CD4⁺ T cells and HTLV-1–specific CD8⁺ T cells may play a role in the disease pathogenesis. Patients with HAM/TSP have high proviral loads despite vigorous virus-specific CD8⁺ T-cell responses; however, it is unknown whether the T cells are efficient in eliminating the virus in vivo. To define the dynamics of HTLV-1–specific CD8⁺T-cell responses, we investigated longitudinal alterations in HTLV-1 proviral load, amino acid changes in an immunodominant viral epitope, frequency of HTLV-1–specific T cells, and degeneracy of T-cell recognition in patients with HAM/TSP. We showed that the frequency and the degeneracy of the HTLV-1–specific CD8⁺ T cells correlated well with proviral load in the longitudinal study. The proviral load was much higher in a patient with low degeneracy of HTLV-1–specific T cells compared to that in a patient with comparable frequency but higher degeneracy of the T cells. Furthermore, in a larger number of patients divided into 2 groups by the proviral load, those with high proviral load had lower degeneracy of T-cell recognition than those with low proviral load. Sequencing analysis revealed that epitope mutations were remarkably increased in a patient when the frequency and the degeneracy were at the lowest. These data suggest that HTLV-1–specific CD8⁺ T cells with degenerate specificity are increased during viral replication and control the viral infection.