Bronchoalveolar lavage and urine samples are obtained from patients on day of life 0, 1, 3, 7, 14, 21, and 28, and every 2 weeks thereafter until the infant is extubated. Serial blood samples are obtained from patients on day of life 0 (cord blood if possible), 1, 3, 7, 14, and 28, and prior to hospital discharge. Infants who require supplemental oxygen beyond 28 days of life will have 3 additional blood samples obtained at 6, 8, and 12 weeks of life. Those infants with established bronchopulmonary dysplasia who are admitted to the hospital at over 4 weeks of age have plasma samples obtained at the time of admission, and every 2 weeks thereafter for a maximum total of 5 samples.