The differences between myocardial infarct (MI) size, amount, and type of conduction fiber injury and outcome in patients with acute anterior or posterior (inferior) MI with or without complete heart block (CHB) were compared. In patients with acute anterior MIs with CHB the infarcts were larger, CHB was more persistent, necrosis involved the more anterior portions of the conduction system, and the terminal event was cardiogenic shock; whereas in the smaller acute posterior MIs with CHB, the CHB was usually transient, necrosis involved the more posterior portions of the conduction system, and terminal arrhythmias were the main cause of death. This is the first quantitative study that documents the larger size of anterior infarcts with complete heart block in comparison with posterior infarcts with complete heart block.