The ECG detection of ST segment depression during maximal treadmill exercise has diagnostic and prognostic significance in coronary artery disease. This study of 200 consecutive patients undergoing Bruce exercise testing for chest pain documents the ability of a transtelephonic ECG system, capable of storing segments of ECG memory during exercise, to reliably record ST segment depression detected during simultaneous exercise testing (sensitivity 79%; specificity 99.6%). Of the three transtelephonic ECG systems tested, the modified precordial V5 snap-on lead most accurately reflected ST segment amplitude and morphology with no "false positives" (specificity 100%) and few "false negatives" (sensitivity 78%). Therefore, this system is a potentially important clinical tool for accurately and conveniently monitoring coronary patients during exercise outside the hospital (e.g., following myocardial infarction) as well as for providing useful clinical information for ambulatory patients with undiagnosed sporadic chest pain.