We amplified and sequenced portions of the human T-lymphotropic virus type I (HTLV-I) (U3), pol, env, and pX provirus regions (1212 bp per person) from peripheral blood lymphocytes (PBL) of two married couples (case 1 and case 2). Both husbands are patients with HTLV-I-associated myelopathy/tropical spastic paraparesis (HAM/TSP), and the wives are asymptomatic HTLV-I carriers. We selected these regions because the LTR and env regions of murine retrovirus models have been involved in determining tissue tropism. In addition, the predominant im-munogenic epitope for HTLV-I-specific cytotoxic T cells obtained from circulating PBL of HAM/TSP patients was localized in the HTLV-I pX region. Our aim was to examine variations in these HTLV-I regions between affected and asymptomatic spouses. In the HTLV-I regions studied, we detected no sequence variation between each couple. These data do not favor the hypothesis that neurotropic mutants of HTLV-I are involved in the pathogenesis of HAM/TSP.