An intraventricular conduction defect complicated 39 out of 489 cases of acute myocardial infarctions. The defect, with the exception of one case, always developed in infarctions in the anterior wall of the heart. Compared with the other acute anterior myocardial infarctions in which the mortality in the Coronary Care Unit was 15%, infarctions complicated by intraventricular conduction defects have on the whole, presented a worse clinical course and a higher mortality rate (56%). It must be stressed that clinical course and mortality rate have been very different in the various conduction defects. The most innocuous conduction defects were the left anterior hemiblock, and, with some reservation, the right bundle branch block too, -- defects in which the clinical course and the mortality rate were not significantly different from those of the other anterior myocardial infarctions not complicated by intraventricular conduction troubles. On the contrary clinical course was more serious and mortality rate higher (50%) in the cases in which the two defects were associated. Myocardial infarctions complicated by left bundle branch block, right bundle branch block with left posterior hemiblock, alternating bundle branch block or 1 degrees and 2 degrees atrioventricular block and bundle branch block, presented an extremely serious clinical course and a very high mortality rate (83%). These differences in the clinical course and in the mortality rate of the various intraventricular conduction defects are explained by the fact that they are associated with myocardial damage of different degree, which is clarified by knowledge of the blood supply of the intraventricular conduction system.