We conducted a serologic survey for antibodies to human immunodeficiency virus types 1 and 2 (HIV-1 and HIV-2) and human T-cell lymphotropic virus Type I (HTLV-I) in 704 Brazilians with the acquired immunodeficiency syndrome (AIDS) or at risk for it. The study population included 70 homosexual men (11 of whom were prostitutes), 58 bisexual men (19 of whom were prostitutes), 101 female prostitutes from three socioeconomic groups, 13 wives of men with hemophilia who were seropositive for HIV-1 antibodies, and 47 blood donors with positive Venereal Disease Research Laboratory tests for syphilis, all from Rio de Janeiro; 86 female prostitutes from two rural towns in Minas Gerais; 133 patients with AIDS from São Paulo; and 196 men with bleeding disorders who were seropositive for HIV-1 antibodies on enzyme-linked immunosorbent assay, from São Paulo and Rio de Janeiro. The prevalence of HIV-1 infection was highest in the homosexual male prostitutes (45 percent), the wives of patients with hemophilia (38 percent), the bisexual men (28 percent), the homosexual men who were not prostitutes (19 percent), and the female prostitutes from the lower class (9 percent). Combined HIV-1 and HIV-2 infection was found in 3 percent of the patients with AIDS and in 1 percent of the homosexual men. The prevalence of HTLV-I infection ranged from 1 percent in rural female prostitutes to 13 percent in HIV-1-positive men with bleeding disorders in Rio de Janeiro. Combined HIV-1 and HTLV-I infection occurred in 1 to 11 percent of some male subgroups. We conclude that in Brazil HIV-1 infection is already well established among homosexuals, bisexuals, and lower-class female prostitutes, with bisexual men probably acting as a bridge between the heterosexual and homosexual communities, that HTLV-I infection is prevalent in groups at risk for AIDS, and that HIV-2 infection has already been introduced into the country.