Ureaplasma urealyticum has been recognized as an important potential pathogen in premature neonates. Reported rates of colonization of the respiratory tract vary. Data on neonatal ureaplasma colonization outside the United States and Western Europe are rare. Therefore we prospectively studied nasopharyngeal and endotracheal colonization in a cohort of 114 preterm and 100 full term infants within 48 hours of birth. The colonization rate was 24% in the premature infants and zero in the full term infants. Bronchopulmonary dysplasia developed in 19% of infants with nasopharyngeal Ureaplasma colonization and in 4.6% of noncolonized infants (P < 0.03). Bronchopulmonary dysplasia developed in 40% of intubated infants with positive endotracheal Ureaplasma cultures and only in 9.8% of infants with negative endotracheal cultures (P < 0.04). Thus Ureaplasma colonization of either the nasopharynx or the trachea was associated with an increased risk for the development of bronchopulmonary dysplasia (relative risk, 4.0 and 4.1, respectively).