Late potentials occurring at the end of or after the QRS complex were recorded from the body surface by means of a high-gain ECG and the signal-averaging technique in 75 patients, within 24 h from the onset of symptoms of acute myocardial infarction. Late potentials were found in 20 patients (26.7%). Out of this group, eight (40%) developed ventricular fibrillation. There was only one incident of ventricular fibrillation among 55 patients without late potentials (1.8%, P less than 0.001). The three-month mortality after myocardial infarction was slightly higher in individuals with late potentials: 15% vs. 11.5%, but this difference was not statistically significant. We conclude that the presence of late potentials recorded in the acute phase of myocardial infarction indicates a risk of ventricular fibrillation, but this was not associated with an increased mortality rate during the three-month follow-up.