The prevalence of ventricular late potentials (LP) in acute myocardial infarction (AMI) and the relationship between the presence of LP and the occurrence of spontaneous sustained ventricular tachycardia (VT) or fibrillation (VE) in the early phase (within 10 days) of AMI were studied. A total of 251 consecutive patients who had an uninterrupted natural history before special treatment formed this study. Sustained VT or VF occurred in 20 patients (8%). LP were recorded in 67 patients (27%). The incidence of LP was significantly higher among patients with VT/VF than among those without VT/VF (12 of 20, or 60%, vs 55 of 231, or 24%, P less than 0.01). The subgroup of patients whose VT/VF occurred within 1 hour of onset of AMI also showed a higher incidence of LP (5 of 9, or 56%, vs 55 of 231, or 24%, P less than 0.05). The results showed that the mechanism of VT/VF in early period including those happened within 1 hour of onset of AMI in man is due to reentry, and that LP can help to identify those high risk patients with a propensity to develop VT/VF in early period of AMI.