Frequency and significance of occult late potentials on the signal-averaged electrocardiogram in sustained ventricular tachycardia after healing of acute myocardial infarction. The quantitative and morphologic characteristics and significance of late potentials on the signal-averaged electrocardiographic QRS complex remain unknown. To assess this, the signal-averaged electrocardiogram of 48 patients (mean age +/- standard deviation 62 +/- 9 years) with sustained ventricular tachycardia (VT) after healing of acute myocardial infarction and late potentials were analyzed. Late potentials could be classified into 3 morphologic subtypes: type I late potentials (19 patients, 40%) occurred in the terminal 40 ms of the QRS complex; type II late potentials (16 patients, 33%) started before the end of the QRS complex and extended 30 +/- 17 ms into the ST segment; type III late potentials (13 patients, 27%) started after the end of the QRS complex in the ST segment and ended 67 +/- 27 ms after the end of the QRS complex. The amplitude of the late potentials in type III, when compared with types I and II, was significantly lower, whereas the QRS duration on the electrocardiogram in type I, when compared with types II and III, was significantly longer. Computer algorithm based on noise failed to identify most type III late potentials. No difference was noted in age, sex, site of the myocardial infarction, and rate of induced VT among the 3 types. 