Fifty-seven patients with electronic pacemakers were studied from electrocardiograms (ECGs), vectorcardiograms (VCGs), and echocardiograms. Thirty-nine patients with transvenous right ventricular (RV) pacemakers showed a left bundle branch block (LBBB) pattern and nine patients with transvenous RV pacemakers showed a right bundle branch block (RBBB) pattern. There was no evidence of perforation of the right ventricle or malposition of the catheter electrode. Eight patients with left ventricular (LV) pacemakers showed a RBBB pattern and one showed a LBBB pattern. The maximal QRS vector of a RBBB pattern induced by RV pacing was directed leftwards and anteriorly, whereas that of a RBBB pattern induced by LV pacing was oriented rightwards and posteriorly. A rapid posterior motion of the left side of the interventricular septum (IVS) during the early systole, and/or anterior or flat motion of the IVS during the ejection period were almost exclusively limited to cases with RV pacing, regardless of the ECG wave form. There was one exceptional case in LV pacing, which showed a LBBB pattern with the same septal motion as that in RV pacing. However, the maximal QRS vector in this case was directed inferiorly, which is in sharp contrast to that in the RV pacing which was directed superiorly. Based on the hypothesis that the ECG wave form by LV pacing might be equivalent to that in a case of perforated right ventricle, the following conclusions can be drawn from the present study. (1) A RBBB pattern in RV pacing could be differentiated from perforation of the right ventricle. The following findings may support uncomplicated RV pacing: (a) the left and anterior orientation of the maximal QRS vector, and (b) a rapid initial posterior septal motion during the early systole and/or a paradoxical anterior septal motion during the ejection period. (2) A LBBB pattern with inferior orientation of the maximal QRS vector would suggest perforation of the right ventricle.