A retrovirus, human T cell lymphotropic virus type 1 (HTLV-1), is an essential but not a sufficient aetiologic factor for tropical spastic paraparesis (TSP). Because some TSP patients have biological false positive tests for treponemal infections (BFP-STS), we used ELISA to study BFP-STS and anticardiolipin antibodies in 42 Jamaican TSP patients. The data indicate that in TSP anticardiolipin antibodies occur in about 26% of patients, are associated with biological false positive treponemal serology, are relatively restricted to the IgA isotype and may be induced by HTLV-1 or other nontreponemal infections.