Forty one of 210 preterm infants ventilated for respiratory distress syndrome in a three year period had radiological evidence of pulmonary interstitial emphysema. The development of this condition was significantly associated with malpositioning of the endotracheal tube in a main bronchus and the use of high peak pressure ventilation. Pulmonary interstitial emphysema was associated with a significant increase in the number of pneumothoraces, intraventricular haemorrhages, and the need for prolonged respiratory support, but did not increase mortality. Although in 12 infants in whom fast rate ventilation was used there was a significant reduction in the number of pneumothoraces, outcome was not altered in any other way. Fast rate ventilation may be of greater benefit if initiated before the development of pulmonary interstitial emphysema.