A cross-sectional study was done to determine the seroprevalence of Mycoplasma penetrans in human immunodeficiency virus (HIV) type 1-seropositive and -seronegative persons recruited in France. The data were analyzed with respect to the sociodemographic, clinical, and biologic status of the patients. M. penetrans seropositivity was associated with HIV infection (18.2% of HIV-seropositive vs. 1.3% of HIV-seronegative persons were M. penetrans-seropositive; P < .001). M. penetrans infection was predominantly but not exclusively associated with homosexual practices in HIV-seropositive subjects and thus presumably sexually transmitted. M. penetrans seroprevalence increased with progression of HIV-associated disease. No association was found between M. penetrans and Kaposi's sarcoma. Thus, there is an unambiguous association between M. penetrans and HIV, particularly among homosexual persons, but its clinical significance remains to be investigated.