There are conflicting reports concerning the prevalence of Mycoplasma fermentans in HIV-positive patients and its association with AIDS. Serum antibodies to M. fermentans were measured by a modified immunoblotting technique in 48 HIV-positive heterosexual patients and in 30 HIV-negative heterosexual controls. Antibodies to M. fermentans were detected in 19 (40%) of HIV-positive patients and in three (10%) of the HIV-negative controls (P = O-01). The prevalence of antibodies to Mycoplasma hominis and to Ureaplasma urealyticum was similar in both groups. In the HIV-positive group, 16/19 (84%) M. fermentans-positive patients developed AIDS, compared to eight of 29 (28%) M. fermentans-negative patients (P = 0.0004). The HIV-positive patients with antibodies to M. fermentans had a lower CD4+ cell count and a higher prevalence of antibodies to the other mycoplasma tested (P=0.007 and P = 0.03, respectively), as compared to the patients without antibodies to M. fermentans. These findings may suggest that the presence of antibodies to M. fermentans indicate an opportunistic infection. Of the 19 M. fermentans-positive patients, 11 were positive on the first examination, and eight became positive during the follow-up period. Seven out of these eight patients developed antibodies to M. fermentans before the development of AIDS. Therefore, the possibility exists that M. fermentans might influence the development of AIDS.