Pulmonary function was measured in 18 children aged 6 to 9 years who had been born prematurely (mean birth weight 1760 +/- 555 g) and who had each received greater than 100 hours (mean 177 +/- 74 hours) of mechanical ventilation for respiratory distress syndrome (RDS). We used as controls 26 children aged 6 to 7 years who had been born prematurely (mean birth weight 1636 +/- 554 g) but who had required no treatment for pulmonary disease. Results for total lung capacity, FEV1, ratios of functional residual capacity and residual volume to total lung capacity, specific airway conductance, and alveolar plateau slope did not differ in the RDS and control groups. Eight of the 18 children in the RDS group had had radiologic evidence of bronchopulmonary dysplasia at 30 days and oxygen dependence at 30 days, but did not differ from the control group for any of the indices of pulmonary function. However, FEV1 and specific airway conductance were significantly reduced in the premature control group compared with children born at term. Therefore, factors associated with prematurity rather than combined effects of RDS and its treatment determined pulmonary function at age 6 to 9 years.