We evaluated lung function in 20 infants with bronchopulmonary dysplasia (BPD) during the first year of life. Compared with a group of age- and size-matched controls, the infants with BPD had a significantly (P less than 0.005) lower functional residual capacity (FRC; 25 +/- 4 vs 18 +/- 6 ml/kg) at less than 10 1/2 months after conception, but no significant difference during the remainder of the first year. The partial expiratory flow volume curves in the infants with BPD were markedly concave, with tidal breathing approaching expiratory flow limitation. The infants with BPD had significantly (P less than 0.01) lower absolute and size-corrected flows than did control infants, and 50% of the infants with BPD required rehospitalization because of acute respiratory distress associated with a lower respiratory tract illness. In addition, the slope of the linear regression of maximal expiratory flow at FRC (in milliliters per second) vs length (in centimeters) was significantly lower (P less than 0.001) for the infants with BPD than for normal control infants (2.25 vs 4.52), indicating poor growth of the airways. Oxygen saturation (SaO2 was negatively correlated with maximal expiratory flow at FRC, indicating that measurement of SaO2 alone may not be sufficient in the evaluation of lung function in infants with BPD. We conclude that, although infants with BPD improve clinically during the first year of life, they have abnormal functional airway growth; the decreased expiratory flow reserve helps to explain their high risk for acute respiratory distress during lower respiratory tract illness.