Alveolar septal fibrosis, the main residual feature in the "healed" stage of bronchopulmonary dysplasia (BPD), was the consistent finding in 28 infants who died at 3 to 40 months of age, all having had moderate to severe BPD in the neonatal period. The cause of death in 68 per cent of the cases was progressive respiratory failure related directly to the residual changes. An additional 18 pe cent of the infants died of pneumonia superimposed on the long-standing healed bronchopulmonary dysplasia. Cardiomegaly was present in 84 per cent of the cases; biventricular hypertrophy was present in 29 per cent of the cases, right ventricular hypertrophy alone in 21 per cent, and left ventricular hypertrophy alone in 21 per cent. Evidence of pulmonary hypertensive vascular disease was found in 68 per cent of the cases. The pulmonary changes of alveolar septal fibrosis are strikingly variable within individual infants, with moderate or severe fibrosis in one area and normally inflated or hyperinflated lung in the adjacent sublobule or lobe. It is postulated that this variability may be related to a protective effect of necrotizing bronchiolitis (a prominent feature of the acute stages of BPD), whereby the occlusion of the bronchioles shields the distal sublobule from the high oxygen tensions and ventilatory pressures used in treating BPD.