Pulmonary function studies were performed in 11 neonatal intensive care survivors both during infancy and later in childhood. Lung function was compared with the respiratory support given in the neonatal period. The mean +/- SE thoracic gas volume was 96 +/- 4% predicted in infancy and rose to 122 +/- 8% predicted during childhood (P less than 0.005). The specific airway conductance (SGaw) in infancy was 57 +/- 7% predicted and rose to 90 +/- 8% predicted in childhood (P less than 0.0025). Abnormalities in SGaw were found only in ventilated infants, and there was a negative logarithmic correlation between the treatment score in the neonatal period and the SGaw in both infancy and childhood. The data indicate a long-term improvement in airway conductance of moderately affected infants with the development of mild hyperinflation in childhood possibly resulting from residual small airway abnormalities despite a symptomless clinical course. The residual abnormalities in prematurely born infants were in proportion to the intensity of treatment required in the neonatal period.