In 13 patients, Korotkoff sounds were recorded from within the brachial artery along with external sounds and pressures. Korotkoff sounds are heard distal to the cuff and not proximal to it and appear at the time of pressure rise in the vessel distal to the cuff. As cuff pressure is lowered from above systole to below diastole in normal patients, both external and internal sounds begin at the systolic level. The external sounds usually disappear above the diastolic level, while the internal sounds disappear at the diastolic level. In patients with aortic insufficiency, the addition of a pistol-shot sound makes the identification of the Korotkoff sounds difficult. Changes in the character of the Korotkoff sounds cannot, therefore, be used as a criterion for diastolic pressure measurement in these abnormal patients. The frequency spread of the Korotkoff sounds approaching diastolic pressure is reduced with the lower frequencies predominating.