Dr. Andrew G. Morrow: Since the clinical program of the National Heart Institute was initiated in 1953, a continuing interest in both the Cardiology and Surgery Branches has been the study and treatment of patients with the congenital or acquired malformations that result in obstruction to left ventricular outflow, that is, aortic stenosis in its various forms. In 1963 an NIH Clinical Staff Conference was devoted to the subject of aortic stenosis (1). In the ensuing 5 years our understanding of the physiologic, pathologic, and hemodynamic characteristics of these malformations has increased manyfold, and the techniques and the results of