This report describes the late potentials obtained by signal averaged electrocardiography (ECG) in 19 patients (13 male, 6 female, mean age 31 +/- 17 years) with acute myocarditis and their evolution. Five patients (26%) had positive signal averaged ECG in time domain analysis and two had technically inadequate signal averaged ECG. All five patients with positive signal averaged ECG, and 6 of the 12 patients with negative, developed heart failure (100%). One patient died. Signal averaged ECG was repeated at 3 months in all 18 surviving patients. Two patients had a good clinical evolution, normalized left ventricular function, and changed from positive to negative signal averaged ECG. All patients with heart failure on admission with negative signal averaged ECG had clinical improvement. Late potentials in patients with acute myocarditis are associated with a worse prognosis and greater extent of myocardial damage.