OBJECTIVE The purpose of this study was to determine the prevalence of Ureaplasma urealyticum (UU) and Mycoplasma hominis (MH) in patients with preterm labor or preterm premature rupture of membranes (PPROM) and to determine the effect of these organisms on pregnancy outcomes based on the density of colonization. METHODS The study group consisted of 184 women with preterm labor or PPROM. Vaginal cultures for UU and MH were performed for all patients at admission, and the placentas were histologically evaluated after delivery. RESULTS The prevalence of positive vaginal fluid cultures for genital mycoplasma was 62.5% (112/179). This group included 99 patients carrying only UU and 13 carrying both organisms. No patients were found to carry only MH. Compared to patients only positive for UU, patients with both organisms showed significantly decreased gestational age at birth and birth weight, and significant increases in the incidences of preterm birth, NICU admissions and histologic chorioamnionitis. CONCLUSIONS Vaginal MH tends to be detected with UU, and patients carrying both organisms simultaneously had more severe adverse pregnancy outcomes compared to patients in preterm labor or PPROM who were only positive for UU.