The effect of postnatal pulmonary artery obstruction on lung growth has not previously been reported. This report describes two patients in whom pulmonary artery banding in early infancy was complicated by complete occlusion of the right pulmonary arter. In each case, the right lung became less voluminous than the left. In one patient, the alevoli of the affected lung were found to be smaller than normal at autopsy. In the other patient, surgical reconstitution of pulmonary arterial flow resulted in a return to normal volume of the affected lung. Observations suggest that pulmonary arterial flow should be restored as early as possible to minimize impairment of lung growth and to avoid irreversible pulmonary hypertension in the opposite lung.