SummaryThe electrocardiograms called bundle branch block mean a trouble of the interventricular conduction, which is not yet located or explained exactly.The classification of these troubles to their electrocardiographic aspects can only have a morphologic value.These troubles are frequently found and probably are more numerous than could be ascertained till now. They are mostly associated witb a sclerosis of the myocard.The statistic study is difficult as there are no systematic and repeated electrocardiographic recordings in the population. However the observations we gathered enable us to state that a bundle branch block only shows an anomaly of the myocard. Its presence has no influence on the prognosis of a heart disease.