We describe a 34-year-old man from southern Florida with a history of intravenous drug use, dually infected with human immunodeficiency virus type 1 (HIV-1) and human T-lymphotropic virus type II (HTLV-II), who developed a myelopathy clinically indistinguishable from HTLV-I-associated myelopathy/tropical spastic paraparesis (HAM/TSP). This myelopathy was characterized by spastic lower extremity weakness, distal paresthesias, sensory loss with a discrete thoracic level to pinprick, back pain, impotence, and sphincter disturbances. Nerve conduction studies revealed an associated mixed axonal and demyelinative neuropathy. Despite a lack of response to 10 months of zidovudine therapy, the myeloneuropathy improved dramatically 2 years after its onset in the absence of any therapeutic intervention.