Evidence of peri-infarction block was found in 39% of the electrocardiograms of 1,938 patients with definite evidence of myocardial infarction. It most commonly occurred acutely or within several days following infarction and was more common with recurrent infarction. Peri-infarction block infrequently disappeared and was associated with higher values for serum glutamic-oxalacetic transaminase and a greater mortality in the first year following myocardial infarction. There was no autopsy correlation of peri-infarction block and ventricular aneurysmal formation.The term "peri-infarction block" is appropriate only when abnormal intraventricular conduction occurs in association with definite electrocardiographic evidence of myocardial infarction.