Bronchopulmonary dysplasia (BPD) is a chronic obstructive pulmonary disease of prematurely born infants following prolonged mechanical ventilation and oxygen therapy. Developmental changes in pulmonary function of children with BPD during their early years have been difficult to study. We longitudinally studied maximal expiratory flow-volume curves by the forced deflation technique in 11 infants who had previous tracheostomy with moderate to severe BPD. Patients were classified into: those who were mechanically ventilated for less than 5 months (Group A), and those who were ventilated for 10 or more months (Group B). At 6 months of age, forced vital capacity (FVC) was 28.1 and 25.5 mL/kg in Group A and B, respectively, significantly less than normal (41.8 mL/kg). The maximum expiratory flow at 25% FVC (MEF25) at 6 months of age was 6.9 and 8.1 mL.kg-1.s-1 in Group A and B, respectively, (predicted value, 39.2 mL.kg-1.s-1). FVC reached the normal range by 12 months of age in Group A, but remained lower until 36 months of age in Group B. MEF25 gradually increased in Group A, reaching 18.0 mL.kg-1.s-1 at 36 months of age, whereas in Group B it was severely decreased at the same age (3.5 mL.kg-1.s-1). More than 75% of the patients had airway hyperreactivity at all ages. We have demonstrated that in patients with moderate to severe BPD, vital capacity is moderately decreased, but catches up to normal levels by 36 months of age. In contrast, severe lower airway obstruction persists in all infants, although in those with moderate BPD gradual improvement is seen. These findings suggest that in BPD neither obstruction of the smaller intrathoracic airways nor bronchial hyperreactivity resolves during the first 3 years of life.