Summary: Human T-cell lymphotropic virus type 1 (HTLV-I) status was assessed in 994 patients attending a sexually transmitted disease (STD) clinic in Kingston, Jamaica, between November 1990 and January 1991 for a new STD complaint. Of 515 heterosexual men, 36 (7.0%) were HTLV-I seropositive, as were 38 (7.9%) of 479 women. HTLV-I seroprevalence increased with age in women. A history of blood transfusion was associated with HTLV-I in both sexes, significantly so in men [odds ratio (OR) 4.7, confidence interval (CI) 1.1-1.7 for men; OR 1.9, CI 0.6-5.0 for women]. Further analysis excluded all persons reporting a transfusion. On multiple logistic regression analysis, independent associations with HTLV-I infection in men were shown for marital status (OR 3.5, CI 1.2-1.0 for married/common law vs. single/visiting unions), agricultural occupation (OR 9.0, CI 2.0-41), bruising during sex (OR 2.9, CI 1.0-8.1),> 15 years at first sexual intercourse (OR 2.9, CI 1.0-8.2), and a positive test for hepatitis B surface antigen (OR 7.3, CI 1.2-52). In women, associations were shown for two or more sex partners in the 4 weeks prior to complaint (OR 4.9, CI 1.8-13), 11 or more lifetime sexual partners (OR 5.9, CI 1.3-27), aged <15 years at first sexual intercourse (OR 2.3, 1.0-5.4), bruising during sex (OR 2.7, CI 1.1-6.6), microhaemagglutination-Treponema pallidum positivity (OR 3.6, CI 1.6-8.4), and human immunodeficiency virus infection (OR 1.4, CI 2.1-9.2). STDs and bruising during sex may facilitate sexual trans-mission of HTLV-I, whereas sexual activity is a more important risk factor in women than men. Programs promoting safer sexual practices and controlling STDs may reduce HTLV-1 infection in Jamaica