Spontaneous myocardial ischemia and the signal-averaged electrocardiogram. The effects of transient myocardial ischemia on the signal-averaged electrocardiogram were investigated in 13 patients with coronary artery disease and spontaneous angina undergoing 3-channel ambulatory electrocardiography. Ischemia was seen as ST elevation in 2 patients or ST depression in 11; it was anterior in 5 patients, inferior in 4 and undefined in 4. Signal-averaged electrocardiograms with noise levels less than or equal to 1 microV were obtained from Holter tapes during 54 of 61 ischemic attacks recorded in the study group (88%), and compared with 54 tracings recorded within 60 minutes of the index attacks. Baseline tracings were normal in 8 patients (62%), showed a long QRS duration in 2 (15%), and both a long QRS duration and a late potential in the remaining 3 (23%). Comparison of recordings at baseline and during ischemic attacks revealed no significant changes in signal-averaged electrocardiographic parameters. Absence of significant differences was also noted when analysis was performed according to the type of ischemic attacks (associated with ST elevation [n = 14] or ST depression [n = 40]), their location (anterior [n = 21] or inferior [n = 23]), their duration (greater than 10 minutes [n = 29] or less than or equal to 10 minutes [n = 25]), and their magnitude (greater than 2 mm [n = 18] or less than or equal to 2 mm [n = 36]). It is concluded that spontaneous transient myocardial ischemia, independent of its type, location, duration and magnitude, does not generate a substrate for late potentials on the signal-averaged electrocardiogram. 