Objective: This study was undertaken to evaluate the frequency of umbilical cord blood infections with Ureaplasma urealyticum and Mycoplasma hominis in preterm 23- to 32-week births and to determine their association with various obstetric conditions, markers of placental inflammation, and newborn outcomes. Study Design: 351 mother/infant dyads with deliveries between 23 and 32 weeks' gestational age who had cord blood cultures for U urealyticum and M hominis had their medical records abstracted, other placental cultures performed, cord interleukin-6 levels determined, placentas evaluated histologically, and infant outcomes determined. Results: U urealyticum and/or M hominis were present in 23% of cord blood cultures. Positive cultures were more common in infants of nonwhite women (27.9% vs 16.8%; P = .016), in women less than 20 years of age, in those undergoing a spontaneous compared to an indicated preterm delivery (34.7% vs 3.2%; P = .0001), and in those delivering at earlier gestational ages. Intrauterine infection and inflammation were more common among infants with a positive U urealyticum and M hominis culture as evidenced by placental cultures for these and other bacteria, elevated cord blood interleukin-6 levels, and placental histology. Infants with positive cord blood U urealyticum and M hominis cultures were more likely to have neonatal systemic inflammatory response syndrome (41.3% vs 25.7%; P = .007; adjusted odds ratio, 1.86; 1.08-3.21) and probably bronchopulmonary dysplasia (26.8% vs 10.1%; P = .0001; adjusted odds ratio 1.99; 0.91-4.37), but were not significantly different for other neonatal outcomes, including respiratory distress syndrome, intraventricular hemorrhage, or death. Conclusion: U urealyticum and M hominis cord blood infections are far more common in spontaneous vs indicated preterm deliveries and are strongly associated with markers of acute placental inflammation. Positive cultures are associated with neonatal systemic inflammatory response syndrome and probably bronchopulmonary dysplasia. © 2008 Mosby, Inc. All rights reserved.