Human T-lymphotropic virus type I (HTLV-I), the causative agent of adult T-cell leukemia and non-Hodgkin's lymphoma (ATLL)--or a cross-reacting retrovirus--has been associated with tropical spastic paraparesis in Martinique, Jamaica, Colombia, Trinidad and Tobago, the Seychelles, and probably also in Zaire. The Caribbean basin and sub-Saharan Africa are endemic for ATLL. A similar etiology has been invoked in a chronic spastic myelopathy occurring in areas of high ATLL endemicity in Japan. An HTLV-I viral antigen has been demonstrated in cerebrospinal fluid lymphocytes of a Japanese patient with myelopathy. Human T-lymphotropic virus type I antibodies have occurred in patients in Florida and Japan (areas of HTLV-I endemicity) who were diagnosed as having clinically definite multiple sclerosis (MS), but not in patients with MS in other parts of the world. Human T-lymphotropic virus type I, like some lentiviruses--visna and human immunodeficiency virus, in particular--may be both lymphotropic and neurotropic. Tropical spastic paraparesis, the Japanese myelopathy, and, perhaps, an MS-like neurologic syndrome, may represent clinical variants of the same disease, a retroviral myelopathy.