In conclusion, we have defined a unique T cell population (THAM cells: IFN-γ+CCR4+Foxp3−CD4+CD25+ T cells) that are rarely encountered in normal individuals, are infected by HTLV-1, and are found to be abnormally increased and proinflammatory and correlate with disease severity in HAM/TSP patients. Here, CCR4 is linked to tropical spastic paraparesis.