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Thirty cases in which transient left anterior hemiblock (LAHB) obscured the diagnosis of right bundle branch block (RBBB) appearing during the first days of an acute myocardial infarction (AMI) are presented. Twenty-eight of the patients with AMI had a clear septal wall involvement, while the remaining two had an anterolateral and lateral wall AMI, respectively. These intraventricular conduction defects developed 2-120 hours (mean 64.9 ± 26 hours) after the acute event, and persisted for 24 hours to 7 days (mean 63.1 35 hours).The ECG was characterized by a pure LAHB with wide QRS complexes and the presence of RBBB was shown by recording high V, and right-sided chest leads. The vectorcardiogram was also useful in several cases.The clinical course of this type of bifascicular block was transient and benign, with an in-hospital mortality of 6.7%. No patient developed trifascicular or complete atrioventricular block and, therefore, we conclude that prophylactic installation of a temporary pacemaker is not indicated in this type of bifascicular block. The possible role of extracellular potassium released during acute myocardial necrosis in the pathophysiological mechanism of these blocks is discussed.