Experimental findings previously observed in dogs demonstrated that block of the posterior subdivision of the left bundle branch of His (LPSB) delays the activation of the posterior portion of the free left ventricular wall and of the posterior portion of the interventricular septum, and gives rise to characteristic ECG and VCG changes. These observations permit the recognition of LPSB in clinical tracings. In this paper a study of 17 clinical ECG and VCG records is presented. The main ECG characteristics of LPSB are: qR or QR complexes in Leads III and aV F, with slurring and/or a notch in the downstroke of the R; sometimes a slurring in the initial portion is also observed. Also there is a delayed onset of the intrinsicoid deflection of the R wave in aV F (45 msec.). In vertical hearts the above features are also observed in Leads V 5 and V 6. ÂQRS is generally situated in the fourth quadrant (between +90 degrees and 0 degrees), although it may be deviated to the right. Frequently the S I-Q III pattern is present. The most important VCG data are: clockwise rotation of the VCG frontal plane ( F), counterclockwise rotation of the VCG horizontal plane ( H), and slurrings of the initial and terminal portions of the curve in the three planes. LPSB can diminish or mask the ECG and VCG signs of a posteroinferior myocardial infarction. Based on experimental observations, it was concluded that in a postero-inferior infarction, the presence of terminal and slurred R waves with a delayed intrinsicoid deflection in Leads III and aV F, even with QRS complexes of less than 0.12 second, is due predominantly to an associated LPSB rather than to peri-infarction block. LPSB may diminish the manifestations of right bundle branch block in aV R. Nevertheless the rsR complexes persist in Lead V 1, while the signs of LPSB are recognizable in Leads III, aV F, and V 6.