OBJECTIVE To review the clinical, epidemiologic, immunologic, and virologic aspects of the chronic myelopathy associated with human T-cell leukemia/lymphoma virus type I (HTLV-I), currently called tropical spastic paraparesis/HTLV-I-associated myelopathy (TSP/HAM). METHODS Studies done after 1985, when TSP/HAM was first recognized, were identified by a computer search using MEDLARS II and CANCERLIT. Additional information was acquired from personal files and bibliographies of existing literature. METHODS A total of 400 articles, 90 book chapters, and 150 abstracts from meetings covering all aspects of HTLV-I and neurologic diseases were critically analyzed, and information from 250 publications was included. RESULTS TSP/HAM is present in most HTLV-I endemic areas, with a prevalence ranging from 5.1 to 128 per 100,000 inhabitants. Up to 20% of patients develop TSP/HAM after transfusion of HTLV-I contaminated blood. Pathologic characteristics indicate a chronic meningomyelitis. The clinical features consist of a chronic progressive spastic paraparesis or paraplegia, sphincter disturbances, and minimal sensory loss. Supraspinal and peripheral nerve involvement is sometimes observed. High titers of HTLV-I-specific antibodies are present in the serum and cerebrospinal fluid. The high level of humoral and cellular immunologic response and the association of TSP/HAM with other immunologic diseases suggest an immune-mediated process. Corticosteroids and immunosuppressor treatment usually result in only short-term improvement. CONCLUSIONS TSP/HAM is a common neurologic disease in many parts of the world. All patients with chronic progressive myelopathies should be tested for serum and cerebrospinal fluid HTLV-I-specific antibodies. Systematic screening of blood donors for HTLV-I is necessary to help prevent the dissemination of the virus and the occurrence of post-transfusional cases.