Pulmonary function tests were performed on 36 very low birth weight neonates (birth weight less than or equal to 1000 gm, gestational age less than or equal to 30 weeks, appropriate size for gestational age) from birth until 8 weeks of age. Tidal airflow was measured by means of a pneumotachygraph, and the transpulmonary pressure changes were determined by the esophageal balloon technique. Pulmonary mechanics and energetics were calculated by the least mean square analysis technique at 1/2, 1, 2, 4, 6, and 8 weeks of age. Compliance was lowest at 2 weeks and subsequently increased linearly (at approximately 0.1 ml/cm H2O/wk). When compliance was based on body weight, however, no change was noted with advancing age. Resistance was greatest at 2 weeks and subsequently decreased. The infants maintained a normal minute ventilation with a slightly increased resistive work of breathing. Despite our ability to wean 53% of the infants to room air by 8 weeks, none of them had pulmonary mechanics considered normal for term infants. Infants with and without bronchopulmonary dysplasia (BPD) had similar patterns of pulmonary development. At 8 weeks of age, the pulmonary mechanics in infants with BPD who had been weaned to room air were comparable to those in infants without BPD, an observation that calls into question the rationale for defining BPD at 4 weeks in this population. These data suggest that such pulmonary function profiles provide evidence of persistent lung parenchymal abnormalities.