Bundle branch block (BBB) patterns are commonly considered to represent conduction block in the corresponding bundle branch. However, several histological and experimental studies demonstrated the possibility of a BBB related to an anatomical lesion within the His bundle. Recently there has been demonstrated, also in clinical setting, the possibility of evaluation whether a left bundle branch block (LBBB) is related to an anatomical within such bundle. For this purpose the selective stimulation of the His bundle has been utilized. A distal simulation of such bundle (which can be evaluated by a stimulus-QRS interval shorter than HV interval) normalizing the QRS complexes compresses an anatomical lesion within the proximal His bundle. Such normalization cannot be attributed to a right septum stimulation since the LBBB pattern should be increased instead of disappearing. The Authors think that an anatomical localization within the His bundle can be evaluated only in LBBB. In fact it is possible that a right bundle branch block pattern, if a septal calcification or fibrosis is present, could be normalized also in the presence of an isoelectric line placed between the artificial stimulus and the QRS complex. There is criticized the concept supported by other Authors that normalization of the QRS complex may be related to a summation effect between the sinusal impulse and that artificially induced. In fact such effect can be easy excluded by analizing the temporal relationship between the ventriculograms registered during His bundle stimulation and the atriograms. The clinical implications of the LBBBs in relation with an anatomical lesion within the His bundle are discussed.