Standard precordial electrocardiogram (ECG) leads detect transmural myocardial ischemia but are of limited use in detecting subendocardial ischemia. An early increase in the pulmonary capillary wedge pressure associated with abnormal wave forms has been noted in patients with coronary artery disease. This study sought to evaluate the usefulness of the pulmonary arterial catheter during coronary artery bypass graft surgery in detecting early myocardial ischemia. Forty patients with progressive angina pectoris undergoing elective myocardial revascularization were studied whenever one of the following signs of myocardial ischemia occurred: (a) ST-segment depression greater than 1 mm or, (b) wedge pressure tracing developed an abnormal AC wave greater than 15 torr, or V wave greater than 20 torr. Forty-five percent of the patients developed signs of myocardial ischemia. Three patients developed only ST-segment depression, five patients had ST-segment depression and an abnormal wedge pressure tracing, and 10 patients demonstrated only abnormal wedge pressure tracings. The abnormal wedge pressure tracings were associated with significant elevations of wedge pressures, central venous pressures, and triple indices. This study demonstrates that changes similar to those described in awake patients during cardiac catheterization occur under anesthesia. Abnormalities in the wedge pressure tracing may occur before the onset of ECG changes, indicating the development of myocardial ischemia. The changes in left ventricular compliance and subendocardial ischemia could not be predicted in advance by either the extent of coronary artery disease of degree of left ventricular dysfunction.