Dilatation of the pulmonary artery occurs frequently in cases of valvular pulmonary stenosis and is often called "poststenotic." We determined the diameter of the pulmonary artery angiocardiographically and compared this with the data obtained by heart catheterization in cases of pulmonary stenosis and of idiopathic dilatation of the pulmonary artery. Features are presented which constitute arguments in favor of Laubry's suggestion that we are dealing with two different abnormalities, namely, a valvular stenosis and a dilatation of the pulmonary artery though hydraulic forces may contribute to the final development of the dilatation.