HTLV-I is a human retrovirus and causes adult T cell leukemia and several inflammatory diseases such as HAM/TSP. The infection occurs via HTLV-I-infected cells and the main transmission rout is mother-to-child infection via breast-feeding. In Japan, total numbers of HTLV-I carriers and HAM/TSP patients are recently estimated to be 1.08 million and 3,600, respectively, which exhibit no reduction in numbers. Although the main lesion is in the thoracic cord of patients with HAM/TSP, the inflammatory regions characterized by mononuclear cells infiltration are disseminated throughout the central nervous system (CNS). The patients show higher proviral load compared to the carries and are frequently complicated with HTLV-I-associated inflammatory diseases in other organs, including uveitis, bronchoalveolitis, arthritis, and Sjögren syndrome. Pathologically, HTLV-I-infected lymphocytes and HTLV-I-specific cytotoxic T lymphocytes infiltrate the CNS from the peripheral blood and induce an inflammation without HTLV-I infection of CNS resident cells, leading to bystander damage in the resident cells. Inhibition of mother-to-child infection via breast-feeding is most important to prevent HTLV-I spread and a treatment to eliminate HTLV-I-infected cells should urgently be established.