This study explores the relation of the presence of peri-infarction block to ventricular late potentials in patients with inferior wall myocardial infarction (MI). The hypothesis was that both the gross peri-infarction block pattern and subtle low-level ventricular late potentials are expressions of conduction abnormality associated with infarction. The consequent question arose whether peri-infarction block may have the same association with sustained ventricular arrhythmias that has been demonstrated in postinfarction patients with ventricular late potentials. Seventy patients with documented Q-wave MI were divided into those with (23) and those without (47) peri-infarction block. Signal-averaged electrocardiograms were obtained. Analysis of the vectormagnitude complex revealed that the total duration of that complex and the duration of terminal potential under 40 microV in the peri-infarction group exceeded that in the group without peri-infarction block (p less than 0.0001). The voltage in the last 40 ms of the vectormagnitude complex was also significantly less in the peri-infarction group (p less than 0.0005). There were 13 instances of sustained ventricular tachycardia, ventricular fibrillation or sudden death occurring subsequent to infarction not associated with the acute ischemic event, 11 of which occurred in the peri-infarction group. The significantly higher incidence of late potentials along with the significantly higher incidence of sustained ventricular arrhythmias in the peri-infarction block on the surface electrocardiogram may provide another marker for identifying persons at increased risk for these arrhythmias subsequent to MI.