Risk for human T-cell lymphotropic virus type (HTLV-I) and human immunodeficiency virus (HIV) infection was evaluated in 100 homosexual or bisexual men from Trinidad. High seropositivity for HTLV-I (15% vs 2.4% in the general population) was linked to duration of homosexuality and numbers of partners, suggesting that HTLV-I, like HIV, can be transmitted by homosexual sex. Forty percent of homosexuals compared with 0.19% of the general population were seropositive for HIV, and sexual contact with US homosexual men and prior history of gonorrhea were major risk factors. The seroprevalence of HIV was three times higher than that for HTLV-I, suggesting that HIV is more efficiently transmitted, especially since HIV appears to have been recently introduced into Trinidad. Altered immune status was prominent in individuals infected with HIV and coinfected with HIV and HTLV-I. Whether HIV/HTLV-I coinfection amplifies clinical effects is a hypothesis that will require further evaluation.