The degree of vascularization of the lungs has long been used as a diagnostic aid in the roentgen interpretation of acquired and congenital heart diseases. It has been observed that in cardiac lesions in which there is pulmonic infundibular or valvular stenosis the peripheral vascularity is decreased, whereas, with intracardiac shunts carrying blood from the left side of the heart to the right side, the volume of pulmonary circulation is increased. The presence of diminished vascularity has served as one of the criteria for the performance of various surgical procedures to increase the pulmonary blood flow in patients with cyanotic heart disease. There are cases in which it is difficult to evaluate the pulmonary circulation on plain roentgenograms of the chest. Poststenotic dilatation of the pulmonary artery, which is frequently associated with pulmonic valvular or infundibular stenosis, may give the impression that the flow is increased, whereas it is usually decreased. The post-stenotic dilatation is limited to the main branches of the pulmonary artery, with the hilar vessels ordinarily appearing normal and the peripheral branches decreased in size. Occasionally the dilatation will extend to the hilar vessels, particularly the left. In small intracardiac shunts and rheumatic heart disease, there is frequently only a very little change in the appearance of the peripheral pulmonary circulation. Because of these difficulties an attempt was made to evaluate separately the appearance of the pulmonary arteries and veins on roentgenograms and to determine if they were differentially involved in different types of heart disease. Anatomy The anatomy of the pulmonary veins, extensively reviewed by Lodge (5) and presented also in standard textbooks of anatomy (2), will be described only briefly. The pulmonary veins return the oxygenated blood from the lungs to the left atrium of the heart. As a rule, two main veins arise from each lung, with a trunk coming from each of the five lobes. The trunks for the right upper and middle lobes may unite before they enter the posterior aspect of the left atrium. Occasionally the three veins on the right side enter the heart separately, or the veins of the middle and lower lobes may unite to form a single trunk. Not infrequently the two left pulmonary veins end in a common opening. At the root of the lung the superior pulmonary vein lies in front of and below the pulmonary artery; the inferior is situated at the lowest part of the hilus of the lung and on a plane posterior to the upper vein. The veins arise from a capillary network upon the walls of the air sacs, where they are continuous with the capillary ramifications of the pulmonary artery, and, joining to-gether, form one vessel for each lobule. These vessels unite and form the larger veins which drain the bronchopulmonary segments.