A transient, intermittent, or permanent disorder of intraventricular conduction was found in 35% of 404 patients with acute myocardial infarction. Patients with acute myocardial infarction and abnormal intraventricular conduction were older than patients with normal intraventricular conduction, but there was no significant age difference between patients with normal and abnormal intraventricular conduction who died. Mortality was only slightly higher in patients with preexisting intraventricular conduction disorder than in patients with normal intraventricular conduction. On the other hand, mortality was remarkably higher in patients in whom abnormal intraventricular conduction developed during the course of acute myocardial infarction than in patients with either normal intraventricular conduction or preexisting abnormal intraventricular conduction. Transvenous cardiac pacing had little impact on mortality, death being due to pump failure in the majority of the patients. This study identifies a group of high-risk patients in whom early aggressive management utilizing newer therapeutic modalities such as mechanical circulatory assistance, emergency aortocoronary bypass, or both, might be profitably applied.