Two-channel ambulatory electrocardiographic (ECG) monitoring is a useful method for detecting transient myocardial ischemia in patients with coronary artery disease. However, the monitoring of only 2 leads may fail to detect a significant number of ischemic episodes. In this study, the additional diagnostic value of a third bipolar chest lead was evaluated by recording a simultaneous 12-lead electrocardiogram and a 3-channel ambulatory electrocardiogram during exercise testing in 223 patients (aged 63 +/- 10 years) with proved or suspected coronary disease. Leads CM5, CM3, and an inferior lead (Y-modified or CMf) were monitored on the ambulatory electrocardiogram. Diagnostic ST-segment depression on the standard electrocardiogram was detected in 98 patients (44%), 94 (96%) of whom also had diagnostic ST-segment changes on the ambulatory electrocardiogram. Two additional patients had ST-segment depression only on the ambulatory electrocardiogram (both in lead CM5). Maximal ST-segment depression and duration of ischemia detected on standard and ambulatory ECG leads were similar in the 94 patients in whom ST-segment changes were detected on both types of ECG monitoring. CM5 was the single lead with the highest sensitivity (89%) in detecting myocardial ischemia. The addition of CM3 to CM5 increased sensitivity to 91%, and the addition of an inferior lead to CM5 increased sensitivity to 94%, particularly improving the detection of isolated inferior myocardial ischemia. The combination of all 3 ambulatory ECG leads had a sensitivity of 96%, an improvement of only 2% compared with the best combination of 2 leads (i.e., CM5 +/- inferior lead).(ABSTRACT TRUNCATED AT 250 WORDS)