Previously, we have reported marked pulmonary inflammation in infants who develop chronic lung disease of prematurity. We revisited these infants who did not have clinical or laboratory evidence of infection and searched for Ureaplasma urealyticum, group B streptococci, and other microbes by reverse transcription-PCR performed on RNA extracted from 93 bronchoalveolar lavage samples. From infants ventilated for respiratory distress syndrome, 6 (gestation, 28 wk; birthweight, 880 g) were positive for U. urealyticum and 11 (25 wk, 800 g) were negative. Five (83%) positive and four (36%) negative infants developed chronic lung disease. Each infant was colonized with either biovar 1 or biovar 2 but not both. U. urealyticum was very weakly detectable in two infants on d 1 but was detected in five of six infants at d 10. Furthermore, pulmonary neutrophils, alveolar macrophages, soluble intercellular adhesion molecule-1, and IL-1␤ on d 10 and IL-6 and IL-8 at d 1 were significantly increased in the positive group. A variety of organisms were identified in six samples between 14 and 21 d of age, but all samples were negative for group B streptococci. Our data suggest that U. urealyticum colonization is associated with the development of pulmonary inflammation in infants who subsequently develop chronic lung disease. CLD remains a common respiratory disorder of preterm infants. Despite significant advances with therapeutic modalities, including the introduction of surfactant, various modes of mechanical ventilation and regular use of antenatal corticosteroids, morbidity and mortality remain high. Many risk factors have been identified and these include mechanical ventilation, oxygen therapy, patent ductus arteriosus, and nosocomial infection. An area that has recently received much attention is antenatal infection (1, 2). Even before the infant is delivered, an inflammatory response has been noted and this inflammation appears to be associated with the development of respiratory disease in the newborn infant (3, 4). One candidate for initiating this inflammation is the mycoplasma Ureaplasma urealyticum (5, 6). This organism has been isolated from the lungs of infants who have developed CLD, but whether U. urealyticum causes acute lung injury in preterm infants or whether the association is with the degree of prematurity remains controversial (7). One approach to confirming or refuting this association is to study the link between the pulmonary inflammatory response, which has now been regularly reported in infants who develop CLD (8 -11), and the presence of U. urealyticum in the lungs of infants who develop CLD.Our previous data have demonstrated that proinflammatory cytokines IL-1 and IL-6, the potent neutrophil chemoattractant IL-8, and soluble adhesion molecule ICAM-1, as well as inflammatory cells, namely neutrophils, are increased in the lungs of infants who subsequently develop CLD (8, 9). These findings have been confirmed by many others (10, 11), and it is now generally accepted that pulmonary inflammation is a r...