To determine the extent to which different electrocardiographic systems account for differences in reported incidence of perioperative myocardial ischemia, the authors simultaneously recorded in 109 patients undergoing coronary artery bypass grafting (CABG) the V5 or modified CMS lead on five ECG systems by means of a specially constructed common V5 lead. The systems included a Spacelabs@Alpha 14 Model Series 3200 ECG Cardule at bandwidths of 0.05–125 Hz and 0.5–30 Hz (a typical operating room monitor), a Marquette@Electronics MAC II ECG at 0.05–40 Hz and 0.05–100 Hz (a standard ECG), and a Del Mar@Holler recorder at 0.1–100 Hz. Relative ST-segment position and Incidence of new ischemia compared to the preoperative ECG were determined in 109 sets of preinduction traces and 877 sets of intraoperative traces. ST-segment position on the three recording systems conforming with the American Heart Association (AHA) low-frequency response recommendations (0.05 Hz) were similar. Compared to the standard ECG, ST-segment position on the Spacelabs at 0.5–30 Hz was consistently more negative. Displacement on the Holter was consistently less negative and less positive. By the 0.1-mV displacement criterion for diagnosis of myocardial ischemia on any one ECG system, 16.5% of patients on arrival and 32.1% of patients intraoperatively suffered new myocardial ischemia. Based on the operating room monitor, arrival and intraoperative ischemia were present in 15.6 and 27.5% of patients, respectively. Ischemia at the same periods was less frequent by the standard ECG system (5.5 and 12.8%, respectively) and least frequent by the Holter recorder (4.6 and 8.3%, respectively). If the criterion for ischemia was reduced to the least measurable ST-segment displacement (0.025 mV), the incidence would have been approximately the same with all ECG systems. These data confirm the higher sensitivity of the operating room monitoring system in diagnosing myocardial ischemia with no apparent loss of specificity. The authors suggest that the criterion of 0.1-mV displacement of the ST segment derived from exercise electrocardiography may be too stringent for perioperative patients with documented coronary artery disease.