The localization of the conduction defect in so-called complete A-V block can be established by electrocardiography only indirectly and unreliably. In principle the region of the atrio-ventricular junction, i.e. the A-V node and the bundle of His, may be involved just as well as the two branches. Analysis of the configuration of the QRS complex in the electrocardiogram is unreliable, particularly in cases with bundle branch block, because any pattern can arise from the appropriate site of a tertiary focus of stimulation in one of the two ventricles. Recording His bundle potentials (His bundle electrogram) enables differentiation. In eleven personal observations and 151 cases collected from the literature of complete A-V block the analysis with this method showed that the so-called A-V block in the majority of cases (63%) is actually due to bilateral bundle branch block. In the other 37% an A-V junctional block was present. This type of block could be differentiated by means of His bundle electrograms into an A-V nodal block and His bundle block.