The central nervous system (CNS) pathology of HTLV-I associated myelopathy or tropical spastic paraparesis (HAM/TSP) is reviewed, based mainly on 12 autopsy cases of Japanese HAM/TSP with a serological confirmation of HTLV-I infection. The essential histopathological feature of HAM/TSP is a chronic progressive inflammatory process heralded by parenchymal infiltration of memory CD4 cells. The inflammation involves both the grey and white matter of the spinal cord, and progresses for more than three years after the onset of neurological symptoms, resulting in preferential degeneration of the white matter. In cases with a history of more than nine years, however, the spinal cord lesions appears degenerative rather than inflammatory. Both the inflammation and the white matter degeneration are most conspicuous in the lower thoracic cord. The lateral funiculus is always and most severely affected. Although the parenchymal tissue degeneration is not confined to any particular long tracts, symmetrical degeneration of the lateral pyramidal tract is evident in all cases. The involvement of the posterior and anterior funiculi is variable and neurons are relatively well preserved. Since evidence for HTLV-I infection in the CNS is limited to detection of proviral DNA by the polymerase chain reaction (PCR) and isolation of the virus from CSF cells, autoimmune nature of the disease is suspected, but is supported by ample evidence for derangements of the host immune system compatible with those of autoimmune diseases. Recent studies on induction of white matter degeneration in the rat with a topographical similarity to human HAM/TSP is also briefly reviewed. However, in the rat disease, inflammatory cell infiltrations are inconspicuous.