WHILE the literature on the pathology of the pulmonary arteries and arterioles in cor pulmonale is extensive, only a few cases of primary changes in the small pulmonary veins and venules leading to hypertension in the lesser circulation have been described. Since the clinical findings in patients with pulmonary vascular obstruction of either type are similar, a definite diagnosis may not be established until cardiac catheterization has been done, or may be proved only at necropsy. To our knowledge cardiovascular studies have been performed in only three of the published cases of pulmonary venous obstruction.1-3 Because this condition is rare in young children and is difficult to distinguish from primary pulmonary artery hypertension, we are describing a 21-month-old child with idiopathic pulmonary venous subintimal fibrosis and severe vascular obstruction proved by necropsy. The symptoms, clinical findings, the electrocardiogram and the results of cardiovascular studies, including cardiac catheterization and selective