A series of our neuropathological studies was reviewed in order to clarify pathogenesis of human T lymphotropic virus type 1(HTLV-1)-associated myelopathy (HAM)/tropical spastic paraparesis (TSP). The essential histopathologic finding was chronic inflammation in which inflammatory infiltrates of mononuclear cells and degeneration of myelin and axons were noted in the entire spinal cord. Immunohistochemical analysis demonstrated T-cell dominance, and the numbers of CD4+ cells and CD8+ cells were equally present in patients with shorter clinical courses. Apoptosis of helper/inducer T-cells were observed in these active inflammatory lesions. Horizontal distribution of inflammatory lesions was symmetric at all spinal levels and was accentuated at sites with slow blood flow in the middle to lower thoracic levels. HTLV-1 proviral DNA amounts were well correlated with the numbers of infiltrated CD4+ cells. In situ PCR of HTLV-1 proviral DNA and in situ hybridization of HTLV-1 Tax gene demonstrated the presence of HTLV-1-infected cells exclusively in the mononuclear infiltrates of perivascular areas. From these findings, it is suggested that T-cell mediated chronic inflammatory processes targeting the HTLV-1 infected T-cells is the primary pathogenic mechanism of HAM/TSP. Anatomically determined hemodynamic conditions may contribute to the localization of infected T-cells and the formation of main lesions in the middle to lower thoracic spinal cord.