CD4+ T cells predominate in early lesions in the CNS in the inflammatory disease human lymphotropic T cell virus type I (HTLV-I)-associated myelopathy/tropical spastic paraparesis (HAM/TSP), but the pathogenesis of the disease remains unclear and the HTLV-I-specific CD4+ T cell response has been little studied. We quantified the IFN-γ-producing HTLV-I-specific CD4+ T cells, in patients with HAM/TSP and in asymptomatic carriers with high proviral load, to test two hypotheses: that HAM/TSP patients and asymptomatic HTLV-I carriers with a similar proviral load differ in the immunodominance hierarchy or the total frequency of specific CD4+ T cells, and that HTLV-I-specific CD4+ T cells are preferentially infected with HTLV-I. The strongest CD4+ T cell response in both HAM/TSP patients and asymptomatic carriers was specific to Env. This contrasts with the immunodominance of Tax in the HTLV-I-specific CD8+ T cell response. The median frequency of HTLV-I-specific IFN-γ+ CD4+ T cells was 25-fold greater in patients with HAM/TSP (p= 0.0023, Mann-Whitney) than in asymptomatic HTLV-I carriers with a similar proviral load. Furthermore, the frequency of CD4+ T cells infected with HTLV-I (expressing Tax protein) was significantly greater (p= 0.0152, Mann-Whitney) among HTLV-I-specific cells than CMV-specific cells. These data were confirmed by quantitative PCR for HTLV-I DNA. We conclude that the high frequency of specific CD4+ T cells was associated with the disease HAM/TSP, and did not simply reflect the higher proviral load that is usually found in HAM/TSP patients. Finally, we conclude that HTLV-I-specific CD4+ T cells are preferentially infected with HTLV-I.