Eighty (32%) of 249 infants weighing less than 1000 g at birth developed pulmonary interstitial emphysema (PIE); its incidence was 42% in infants at 500-799 g, 29% at 800-899 g and 20% at 900-999 g. Fifty-five per cent occurred in the first 24 h and 43% were associated with other forms of air leak. The incidence and mortality of PIE did not improve between 1977 and 1984. The mortality rates in infants with or without PIE were not significantly different in the first 4 years of the study period (53% vs 45%). As the mortality improved in infants without PIE during the second 4 years, the difference in mortality rates in infants with or without PIE became significant (68% vs 29%). PIE continues to be associated with serious mortality and morbidity in extremely low birthweight infants. Effective measures to prevent PIE are required before further improvement in their outcome can be achieved.