Fourteen cases of severe aortic insufficiency are presented in which an extremely elevated left ventricular pressure exceeded left atrial pressure during diastole. In 13 of the 14 cases there was proof, or strong suggestive evidence, that the aortic insufficiency was sudden in onset. In nine cases the left ventricular pressure equaled aortic pressure by end-diastole. In five cases left ventricular pressure failed to equal aortic pressure by end-diastole. The similarities between the hemodynamics of experimental and clinically occurring sudden severe aortic insufficiency, including possibly the effect of superimposed mitral insufficiency, was at times striking. The clinical picture in these cases was sufficiently distinctive that the abnormal hemodynamics could be predicted and the presence of sudden, severe aortic insufficiency suggested. The absence of the first heart sound at the cardiac apex, the presence of an early diastolic sound and “double diastolic apex beat,” the harshness of the aortic systolic and diastolic murmurs, all served to make auscultatory differentiation of systole and diastole at times extremely difficult. This difficulty in differentiating the phases of the cardiac cycle by auscultation alone was in itself rather characteristic of sudden severe aortic insufficiency. The poor prognosis in these cases makes early recognition mandatory and early surgery advisable.