From a case of congenital diaphragmatic hernia the pattern of growth of the airways, alveoli, and pulmonary arteries of both lungs, each hypoplastic, has been analysed quantitatively. The impairment of growth for each type of structure is not necessarily the same and differs in each lung. Airway and alveolar numbers are both greatly reduced, although the latter are more nearly normal when related to the number of terminal bronchioli in the lung. In each lung the size of the pulmonary artery at the hilum is appropriate to the lung volume but small for the age of the child. Muscle is found in much smaller arteries than is normal but not to a more peripheral level.

The way the lungs in a case of congenital diaphragmatic hernia might grow after surgical correction of the hernia is discussed and a plea made for respiratory physiological studies in such cases.