The existence of sclerosis of the pulmonary arteries and its secondary effects on the right heart have long been recognized. These changes may depend on the circulatory disturbances in the lungs, due to extensive adhesive pleurisy, emphysema, fibroid changes in the lungs, but, above all, to chronic passive congestion of the lungs induced by left-sided heart lesion, especially mitral stenosis. That there exists, however, a primary sclerosis of the pulmonary arteries with associated right heart hypertrophy has not been generally recognized. So far as I am aware, no case has been published in the English literature, and only a few cases have been reported abroad. This fact seems to justify the publication of a case which came under my observation while I was engaged in study at the Pathologic Institute in Munich during the summer of 1908. I shall also take occasion to review briefly the cases