Data indicate the presence in the Seychelles Islands of a high level of human T cell leukemia/lymphoma virus type I (HTLV-I) endemicity as well as the presence of tropical spastic paraparesis/HTLV-I-associated myelopathy (TSP/HAM). We present here the results of an hospital survey performed since 1988 in La Réunion Island, located in the Indian Ocean southeast of the Seychelles archipelago, aimed at evaluating HTLV-I endemicity, detecting HTLV-I-associated diseases, and characterizing viral isolates. Seven individuals were found to have HTLV-I-specific antibodies in their sera. These include 3 of 257 patients from St. Pierre Hospital, 1 of them exhibiting a typical clinical feature of TSP/HAM (the first described case in this region), 1 blood donor of 3900, and 3 relatives. A further nine individuals exhibiting only "gag-encoded proteins" by Western blot (p19 and/or p24 bands) were found negative by polymerase chain reaction using LTR, pol, and tax HTLV-I specific primers. A long-term T cell line, designated Mel.J, exhibiting T cell activation markers (CD4+, CD25+, HLA-DR+), and producing HTLV-I antigens and viral particles, was established from one of the HTLV-I,-seropositive patients. The sequence of a 522-bp fragment corresponding to the carboxy terminus of gp46 and the majority of gp21 were determined for five HTLV-I-seropositive individuals, including the TSP/HAM patient. Alignment and phylogenetic comparison of these five nucleotide sequences with all the 53 other available HTLV-I env sequences demonstrated that the virus from La Réunion Island belongs to the group of the HTLV-I cosmopolitan subtype and is not related to the Melanesian HTLV-I variants.