The polymerase chain reaction (PCR) was evaluated retrospectively for its ability to detect Ureaplasma urealyticum and Mycoplasma spp. in respiratory tract specimens obtained from adult patients with AIDS. Mycoplasma DNA was detected in specimens from 12 of 84 patients. Of the 107 specimens tested, 13 and seven positive PCR results were obtained with the genus- and species-specific oligonucleotide primers used, respectively, in two different steps. With the latter, one sample was positive for U. urealyticum plus M. hominis, another for M. fermentans plus M. salivarium, and five others were positive for M. salivarium. The unexpected detection of U. urealyticum DNA in respiratory secretions from an adult AIDS patient suggested that this urogenital mycoplasma could have a role in determining or exacerbating respiratory tract infections in the HIV-positive population, but that its low rate of isolation could be related to the frequent failure of methods used currently to detect mycoplasmas