The origin of an audible immediate diastolic murmur in 10 of 22 patients withatrioventricular defects was investigated by intracardiac phonocardiography. It was shown to arise at the site of the atrial septal defect, and all other possible causes of such a murmur were excluded.The production of the murmur depended on the combination of mitral regurgitation and an atrial septal defect of such a size as to permit an interatrial pressure gradient in early diastole.