To study the predictive value for preterm delivery of colonization of the cervix and vagina by ureaplasmas and other potentially pathogenic microorganisms.Prospective analysis of a study group of 200 pregnant women with preterm labor and intact membranes, and a control group of 50 pregnant women. The subjects in both groups were between 24 and 34 weeks of gestation. Ureaplasma spp. and Mycoplasma hominis endocervical cultures were performed for both groups.In the study group, 70 of the women delivered preterm, and all women in the control group carried their pregnancies to full term. Cervical cultures for Ureaplasma spp. were positive in 119 women, and were isolated more frequently in the study group (51.5%) than in the control group (32%), and more frequently in subjects in the study group who delivered preterm (65%) than in those who had a full-term delivery (45%), differences that were statistically significant.Detection of Ureaplasma spp. in the endocervix has a statistically significant relationship to preterm labor (p = 0.03) and preterm delivery (p = 0.02) in pregnant women with preterm labor and intact membranes.