OBJECTIVE The purpose of this study was to evaluate the prevalence of microbial invasion of the amniotic cavity at the time of preterm cesarean delivery for therapy-resistant preterm labor or preterm premature rupture of membranes, which are events that commonly are induced by infection, and to compare this group of patients with a group of patients who underwent preterm cesarean delivery for indications other than preterm labor or preterm premature rupture of membranes. METHODS We studied 207 consecutive women between 23 and 34 weeks of gestation who underwent cesarean delivery. These patients were divided into 3 groups according to the indication for cesarean delivery: patients with preterm labor (group 1), patients with preterm premature rupture of membranes (group 2), and patients with other indications (group 3). In the course of the surgical procedure, amniotic fluid, amniotic membrane, and placental tissue specimens were collected for the detection of pathogens. RESULTS Ureaplasma urealyticum was detected in 43.9% (58/132) of the patients of groups 1 and 2, with no significant difference between these 2 subgroups. In group 3, which served as the comparison group, Ureaplasma urealyticum was isolated in only 2.7% (2/75) of the patients. Ureaplasma urealyticum as a single pathogen was more frequent than all obligate pathogens together (43.9% vs 39.3%). CONCLUSIONS Our results provide evidence for an association between intrauterine colonization with Ureaplasma urealyticum and both therapy-resistant preterm labor and preterm premature rupture of membranes.