The incidence of pulmonary air leak in 230 infants with a birth weight of 500-999 g who were ventilated was 41%. Pulmonary interstitial emphysema occurred in 35%, pneumothorax in 20%, pneumomediastinum in 3%, and pneumopericardium in 2%. The survival rates in those with or without pulmonary air leak were not significantly different in the first four years of the study period (46% v 53%). As the survival improved in infants without air leak during the second four years the difference in survival rates in infants with or without air leak became significant (30% v 71%). Effective measures of preventing pulmonary air leak are required before further improvement in the outcome of these extremely low birthweight infants can be achieved.