Forty six of 142 infants weighing less than 1500 g at birth, who had chest radiographs in the first 5 days of life, developed pulmonary interstitial emphysema (PIE) and in 19 this occurred in the first 24 hours. PIE was seen more frequently in infants weighing less than 1000 g at birth (24 of 57) than in those weighing 1000-1500 g (22 of 85). Ventilation for hyaline membrane disease was strongly associated with PIE, and only babies who were resuscitated, or ventilated, or had hyaline membrane disease developed the disorder. Most pneumothoraces were preceded by x-ray appearances of PIE (17 of 21). Mortality was increased in ventilated infants who developed PIE and was high in those with severe x-ray changes.