A review of 9401 consecutive live births at the Mercy Maternity Hospital, Melbourne, was performed to determine the incidence of air leak in those with respiratory distress syndrome. Respiratory distress was detected in 552 (5.9%) infants and hyaline membrane disease was the most common cause occurring in 238 (2.5%) infants. Air leak developed in 22% of infants with respiratory distress, 8% had pulmonary interstitial emphysema alone, 14% had pneumomediastinum or pneumothorax and 7% had emphysema with pneumomediastinum or pneumothorax. Mortality increased from 12% in infants without air leak to 31% (p < 0.001) in infants with air leak. Ninety-five percent of air leak developed in infants with hyaline membrane, and these were smaller, less mature and sicker than those without air leak. Eighty-seven per cent of air leak developed in infants treated with assisted ventilation and was commoner with mechanical ventilators with a more rapid rise in inspiratory pressure.