The prevalence of human herpesvirus 8 (HHV-8; Kaposi's sarcoma [KS] herpesvirus) infection was determined by IFA in 297 persons living in Brazil and Colorado. The prevalence of antibody to HHV-8 in human immunodeficiency virus (HIV) type 1-seropositive gay men with and without KS was similar in Brazil and Colorado. In Brazil, the prevalence of HHV-8 antibody was significantly greater in HIV-1-seronegative gay men than in HIV-1-seronegative male intravenous drug users. HHV-8-seropositive Brazilian gay men who had a clinical diagnosis of KS or who were infected with HIV-1 had significantly higher titers of HHV-8 antibody than did HHV-8-seropositive, HIV-1-seronegative Brazilian gay men. These findings provide further support for the association between HHV-8 infection and KS and suggest that, as in the United States, HHV-8 infection is transmitted sexually in Brazil.