In Rio de Janeiro (RJ) most cases of paraparesis of obscure origin are associated with the human T-cell lymphotropic virus type I (HTLV-I). Thirty-four consecutive patients with HTLV-I-associated myelopathy/tropical spastic paraparesis (HAM/TSP) from RJ were evaluated. Most patients came from low socio-economic levels. There was no difference in terms of gender. The main affected racial group was white. A history of sexually transmitted diseases was a major risk factor for HAM/TSP and a positive serology for syphilis was found in 26.5% of the patients. The major clinical findings were of a spastic paraparesis with generalized brisk tendon jerks and bilateral Babinki's sign. Sensation was abnormal in 25 patients (73.5%) and five (14.7%) had a sensory level. Three patients (8.8%) had optic atrophy. The cerebrospinal fluid showed a lymphocytic pleocytosis with a mean total protein content of 0.4 g/litre, and an increased intrathecal IgG synthesis in 59.4% of patients. HAM/TSP and multiple sclerosis (MS) occur indigenously in RJ and some HAM/TSP cases can be sometimes confused with MS. Therefore we propose that, in places where MS coexist with HAM/TSP, HTLV-I antibodies should be sought routinely in those MS suspected cases with prominent spastic paraparesis.