The design was that of a retrospective, case-control study. The populations of babies studied in the Phenobarbital Study and the Antenatal Risk Factor Study were not identical and therefore, were not combined for analysis but allowed hypotheses generated in one sample to be tested in the other. In a review of medical records, cases were chosen based on the clinical criterion of a requirement for supplemental oxygen for 28 days or more. An additional requirement for case status was the presence of classical stages II, III, or IV radiographic findings of bronchopulmonary dysplasia/chronic pulmonary disease of prematurity. Positive readings by two independent pediatric radiologists were required for case assignment. Control infants were those who failed to meet case criteria. Hypothesis testing proceeded from univariate to stratified analyses and, finally, to multivariate modeling. Candidates for multivariate analysis included: antenatal factors such as maternal age, race, marital status, obstetric history and pregnancy history; perinatal factors such as birth weight, gestational age, degree of illness; neonatal factors such as respiratory condition, body fluids, and miscellaneous laboratory data.