Until recnetly, coronary arterial perfusion was one of the best methods to protect myocardium during aortic valve replacement. However, the insertion of perfusion cannulas may produce immediate traumatic lesions and late stenosis of the coronary arteries, with grave consequences. Two patients with normal coronary arteries prior to operation developed obstruction of the maximal left main coronary artery following aortic replacement. One of these patients represents the first case of iatrogenic coronary arterial stenosis in which the aortic valve was replaced with a porcine bioprosthesis. Accelerating angina pectoris and ventricular arrhythmias were the presenting clinical manifestations. Aorta-coronary bypass grafting to the left anterior descending and circumflex coronary arteries was successfully performed in one patient, while the other patient died before investigative procedures could be undertaken. Any patient whose aortic valve has been replaced and who develops angina pectoris a few months after operation should be suspected of having developed stenosis of the proximal coronary artery. Coronary angiography should be perfomed promptly. Once the lesion is recognized, the operaiton should be performed posthaste because these lesions are life threatening owing to their proximal location and rapid evolution.