Quantitative analysis has been used to assess growth in a lung from an infant aged 21/2 months in whom a diaphragmatic hernia was repaired at birth. The lungs had been abnormally small at birth but at 21/2 months were of normal volume. Alveoli had multiplied at the normal rate after birth but had not reached the number normal of age. The number per acinus was normal but the alveoli were increased in size, particularly in the left lung. The airway number, and thus alveolar, acinar, and arterial number, were all reduced in both lungs, the ipsilateral being most affected. The pulmonary blood vessels in both lungs showed an increased muscularity that did not correlate with lung volume or alveolar number, a feature that may have been present at birth. The degrees of hypoplasia in the two lungs were different at birth and this difference had been maintained. The effect of the disturbance to lung growth on the functioning of the lung is discussed.