From August 1985 through January 1986, 125 homosexual or bisexual men from the Kingston area were enrolled in a study to evaluate risk factors for infection with human immunodeficiency virus (HIV) and human T-lymphotropic virus type I (HTLV-I). Twelve men (10%) were seropositive for HIV and 6 (5%) for HTLV-I; 1 man had possible coinfection with HIV and HTLV-I. One third of the men reported having had homosexual encounters with foreign visitors or while travelling outside Jamaica, and sexual contact with men in the U.S. was weakly associated with HIV infection (p = 0.11). The median number of partners was 12 per year (range 0-135) and a greater number of homosexual partners per year was associated with HIV seropositivity (p = 0.01). HIV seropositives also were more likely to have a history of lymphadenopathy (p = 0.07). For HTLV-I, there were no obvious risk factors identified, and age-adjusted seroprevalence was not significantly higher than that of heterosexual men. Compared to studies of homosexual men in the U.S. prior to the advent of extensive AIDS education, the Jamaican homosexual population was more sexually conservative. Despite this circumstance, HIV appears to have entered this population via sexual contact with foreign men and spread efficiently among men with a greater number of sexual partners. The frequency of bisexuality (65/125 men) and the 11% HIV prevalence in bisexual men suggest that secondary infection of female sexual partners may occur.