Antibodies to Mycoplasma penetrans were found at an unusually high frequency in male homosexuals with AIDS (55 of 149; 37%) and in human immunodeficiency virus (HIV)-infected asymptomatic homosexuals (13 of 49; 26.5%) but not in intravenous drug users (3 of 308; 1%) and hemophiliacs (1 of 165; 0.6%) with or without HIV-1 infection. Thus, both M. penetrans and Kaposi's sarcoma (KS) occur primarily in male homosexuals and rarely in other groups of patients at high risk of AIDS. Among 414 HIV-1-infected patients, statistical analysis revealed those with M. penetrans antibody were 11.7 times more likely to develop KS. Furthermore, among 198 HIV-infected homosexuals (149 with AIDS and 49 without AIDS), those with KS had M. penetrans-specific antibody at a significantly higher frequency (28 of 47; 59.6%) than did those without KS (27 of 102 with AIDS [26.5%] as well as 13 of 49 without AIDS [26.5%]; odds ratio = 4.1, P < .001). M. penetrans is apparently transmitted sexually through homosexual activity and is epidemiologically linked to formation of KS in homosexual men with AIDS. Parallel tests with M. genitalium revealed no similar link to KS in the same study sample.