The aim of this study was to determine the extent to which bronchopulmonary dysplasia (BPD) affects the diffusing properties of lung tissue in childhood. Pulmonary function in 31 prematurely born children (BW. < 1250 g) was examined at ages 7-11 years. Twenty out of 31 prematurely born children met the criteria for BPD. The remaining 11 children had milder forms of neonatal lung disease. Twenty healthy children of the same age and born at term served as a control group. The diffusing capacity of the lung for carbon monoxide (DLCO) was measured by the single breath method. Lung volumes were determined in a body plethysmograph and expiratory flow rates with a flow/volume spirometer. DLCO values of children with histories of BPD did not differ significantly from those of the prematurely born children without BPD. However, DLCO values in both prematurely born study groups were significantly lower than those in controls born at term. Thoracic gas volumes measured with a body plethysmograph were similar in all groups. Spirometry demonstrated reduced flow rates in both BPD and non-BPD prematurely born children. The results suggest that some structural changes in lung tissues and airways persist for years in children who are born very preterm regardless of whether they develop BPD or not.