The CNS pathology of 10 autopsy cases of Japanese HAM/TSP patients with a serological confirmation of HTLV-I infection was reviewed. The essential histopathological feature was a chronic progressive inflammatory process with marked parenchymal exudation of lymphocytes and monocytes into both the grey and white matter of the spinal cord, uniquely perpetuating for more than 3 years after the onset of neurological symptoms, and resulting in severe degeneration of the white matter accompanied by marked glio-mesenchymal tissue reactions. Both the inflammation and the white matter degeneration were most conspicuous in the lower thoracic cord. The lateral column was always and most severely affected. Although the parenchymal tissue degeneration was not confined to any particular long tracts, symmetrical degeneration of the lateral pyramidal tract was evident in all cases. Diffuse myelin pallor was also seen in the anterior column but it was usually mild. The posterior column was commonly involved but the severity and extent of the white matter degeneration were variable. Neurons were relatively well preserved. These histopathological features of HAM/TSP in Japan largely agree with those previously described for HAM/TSP in tropical regions. In the absence of detectable amount of HTLV-I antigens at the sites of inflammation, "chronic progressive parainfectious myelitis" seems to be the most appropriate descriptive term for this unique histopathology of HAM/TSP.