Thirty-seven patients with acute myocardial infarction complicated by atrioventricular or bundle branch block, or a combination of both, has His bundle electrogram studies performed during their stay in the coronary care unit. The acute mortality of the 14 patients with a complicating bundle branch was 50%. Pump failure was the main cause of death. Three patients in this group had a prolonged H-V interval and one patient had a split blundle of His. The presence or absence of a prolonged H-V interval did not affect mortality in this group of patients. The acute mortality of the 16 patients with an inferior wall myocardial infarction was 6%. The H-V interval was normal in all but one of these patients. The atrioventricular block was caused by a proximal block in all cases. The acute mortality of the seven patients with an anterior wall myocardial infarction was 29%. The H-V interval was prolonged in two of seven patients. Pump failure was the acute cause of the deaths. The presence or absence of a prolonged H-V interval did not affect mortality in this group of patients.