Hemodynamic studies in 12 patients with isolated severe aortic regurgitation, and an apical diastolic rumble (Austin Flint murmur) showed a characteristic pressure tracing in diastole. Simultaneously recorded direct or indirect left atrial and left ventricular pressures showed that left atrial pressure was significantly higher than left ventricular pressure in early diastole and that at end diastole, the ventricular pressure was higher than atrial pressure. This pressure phenomenon was not observed in five patients with equally severe aortic regurgitation who did not have an apical diastolic rumble. The left ventricular end diastolic pressure was markedly elevated in all 12 patients with the Austin Flint rumble. The possible relationship of these hemodynamic findings to the genesis of the Austin Flint murmur is discussed. © 1969.