Some of the morphologic changes of bronchopulmonary dysplasia are frequently found in infants dying of the infantile respiratory distress syndrome, so the frequency of BPD is more frequent as determined morphologically than as determined clinically. Morphologic differences have been described between the lesions of oxygen poisoning and those of BPD, but there are probably no true qualitative differences and such differences as exist are quantitative ones. Clinical and functional studies suggest that lung damage in infancy and early childhood may result in residual functional abnormalities in older childhood, and to a predisposition to chronic airflow obstruction in later life. It is hypothesized that this may be brought about by alteration in postnatal lung growth. Two patients who had an operation for diaphragmatic hernia on the first day of life and who died 8 and 64 months later were used to illustrate this point, since both had obviously abnormal lung structure at the time of death.