We describe postmortem findings in a patient with human T lymphotropic virus type I (HTLV-I)-associated myelopathy (HAM). The patient developed the disease 8 years after blood transfusion and showed good response to corticosteroid treatment but died of cardiac failure. Histologically, chronic, mild meningoencephalomyelitis was noted predominantly involving the bilateral lateral and anterior columns of the middle to lower thoracic segments. The spinal cord lesions showed obvious loss of myelinated nerve fibers and fibrillary gliosis with minimal inflammatory cell infiltration. Electron microscopy of the lesion revealed disintegration of the myelin sheaths, regular separation of the minor dense line of the myelin sheaths, and completely demyelinated axons. In addition, remyelinated fibers with thin central myelin sheaths and disproportionately large axons were seen frequently. These findings indicate that primary demyelination and remyelination by oligodendrocytes occur in the spinal cord lesions of HAM.