Human T cell leukemia/lymphoma virus type I (HTLV-I) isolated in 1980, is a human retrovirus. This CD4+ lymphotropic type C retrovirus is endemic in South Western Japan, the Caribbean region and Africa where it is associated with a rare form of lymphoproliferative disease, adult T cell leukemia. Recently, HTLV-I antibodies has been found in sera and CSF from patients with tropical spastic paraparesis (TSP), a chronic neuromyelopathy of unknown etiology common in tropical areas and in Japanese patients with a similar clinical myelopathy. These data suggest that HTLV-I or an antigenically related virus might be neurotropic or neurovirulent and etiologically linked to such chronic neuromyelopathies. The fact that TSP affects about 10 to 100 persons/100,000 in tropical HTLV-I endemic areas, a prevalence comparable to that of multiple sclerosis in temperate regions, increases considerably the public health interest in HTLV-I and associated diseases. The possible neurotropism or neurovirulence of this retrovirus is discussed.