The aim of this study was to determine the optimal time for recording the signal averaged electrocardiogram in order not to miss cardiac events after acute myocardial infarction. Three signal averaged electrocardiograms were recorded by Simson's method in the early post infarction period at Day 1 (24 to 48 hours after the onset of symptoms) at Day 8 (8 days after the onset) and Day 15 (2 weeks after the onset) in 66 patients. The results showed late potentials in only 18% of patients at Day 1 and that this was not related to a greater risk of ventricular arrhythmia in the acute phase. Late potentials were recorded in 34.8% of patients on Day 8 and 28.7% at Day 15. We suggest that signal averaged electrocardiography be performed one to two weeks after the onset of myocardial infarction; 13 of 66 patients had abnormalities at Day 8 but not on Day 1.