Serial signal-averaged electrocardiograms (ECGs) were performed every 48 hours in 50 patients admitted to the coronary care unit with acute myocardial infarction. The prevalence of late potentials was 32% at presentation (mean time to recording 12.4 +/- 6.6 hours after onset of chest pain) and increased progressively throughout the hospital stay. New late potentials were recorded in patients with no prior acute myocardial infarction as early as 3 hours after the onset of chest pain and as late as 8 days. Late potentials appeared transiently in only 3 patients. The detection of late potentials in the initial signal-averaged ECG identified patients with clinically significant early ventricular arrhythmias with a sensitivity of 80% and specificity of 72%. The predictive accuracy was 38% for a positive test and 94% for a negative test. Patients with early ventricular arrhythmias had significantly lower voltage in the terminal 40 ms of the filtered QRS complex (16 +/- 8 vs 32 +/- 19 microV, p less than 0.01) than those without arrhythmias. The signal-averaged ECG may be useful in identifying patients at high risk of developing clinically significant early ventricular arrhythmias after acute myocardial infarction.