To evaluate the physiologic course of pulmonary function in infants with bronchopulmonary dysplasia (BPD) weighing less than 1,250 g at birth, 24 infants with BPD underwent serial pulmonary function evaluations from birth until 2 yr of age. All infants were intubated at birth and the mean duration of mechanical ventilation was 38 +/- 4 d. Passive respiratory system compliance (Crs) and resistance (Rrs) were measured between 10 and 20 d of life during mechanical ventilation. Thereafter pulmonary mechanics and functional residual capacity (FRC) were evaluated at 3, 6, 9, 12, and 24 mo of postnatal age. Forced expiratory flow (Vmax,FRC) was measured at 2 yr of age. A severe alteration on Crs (50% of predicted) was found during the acute phase of BPD, associated with abnormal values of Rrs. A progressive improvement (ANOVA, p < 0.0001) occurred in the first year of life and, at 24 mo of age, Crs and Rrs reached the range of normalcy. FRC value was 86 +/- 7.5% of predicted at 3 mo and gradually increased to a mean value of 115 +/- 5% of predicted at 2 yr of age. In spite of the good resistance time course over the 2-yr evaluation, less favorable data of Vmax,FRC were found with individual values reduced more than 40% of predicted values in 70% of the children. In conclusion, although pulmonary mechanics of BPD survivors improves during the first years of life, reaching the range of normal values, at 2 yr of age they still present a substantial airway function impairment as revealed by the low forced expiratory flows.