To gain insight into the prognostic significance of the presence of late potentials detected by body surface signal-averaged electrocardiography during acute myocardial infarction, the authors prospectively studied the relationship between the presence of late potentials and the occurrence of ventricular arrhythmias and sudden death in 52 patients from the initial presentation of myocardial infarction to 14 months after discharge from the hospital. Signal-averaged recordings were performed on admission and again at the time of discharge (mean, 12 days from initial tracing). Late potentials were present on the first recording in 10 patients, 6 of whom had late potentials on the second recording. Twelve patients had late potentials on the second recording, and six of these did not have late potentials on the first recording. Early in-hospital sustained ventricular tachycardia and ventricular fibrillation occurred in three patients, all in the late potential-negative group. There were 10 deaths, 7 in the hospital and 3 late. There was no significant difference in early or late mortality between the late potential-positive and the late potential-negative groups due to the small sample size, although there was a trend toward higher mortality in the late potential positive group. Late potentials detected by body surface signal-averaged electrocardiography did not predict the occurrence of early in-hospital ventricular tachycardia or fibrillation or sudden death. During the acute phase of myocardial infarction, late potentials are often variable and transient findings on signal-averaged electrocardiography.