Human T-cell lymphotropic virus type I (HTLV-1) is a human retrovirus and the etiologic agent for a progressive neurological disease called HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). With its primary site of involvement occurring at the thoracic spinal cord level, HAM/TSP is characterized by muscle weakness, hyperreflexia, lower extremity spasticity, and urinary disturbance. However, HAM/TSP has also been associated with other autoimmune disorders as well as widespread dysfunction within the nervous system that results in less frequent symptoms of cerebral, cerebellar, and cranial nerve involvement. Although masked by the more prominent myelopathy, involvement of the peripheral nervous system also occurs. Syndromes of anterior horn cell, peripheral nerve, and muscle dysfunction have been reported.