Twenty-one patients with transvenous endocardial right ventricular pacemakers and one patient with epicardial right ventricular pacemaker inducing artificial left bundle branch block (LBBB) were studied with echocardiographic and vectorcardiographic techniques. Sixteen patients were found to have an initial very active posterior motion of the interventricular septum occurring within 70 msec. (range, 40 to 100 msec.) of the pacemaker artefact followed by posterior movement during the ejection period (Pattern A). Eighteen of 20 patients with clinical LBBB serving as a control group showed a dynamic posterior motion occurring within 40 msec. of the onset of QRS and preceding anterior (paradoxical) motion of the septum during ejection. In four patients, following the initial brief posterior septal movement, there was flat or anterior movement of the septum during the ejection period (Patern B). Two patients had myocardial infarction and one had atrial septal defect and epicardial right ventricular pacemaker. Only in two patients the initial brief posterior septal motion was not recorded before moving posteriorly during the ejection period (Pattern C). The different patterns of septal motion found in patients with artificial LBBB and in those with natural LBBB could be explained by differences in activation of the heart, as shown by vectorcardiography. Echocardiographic septal evaluation of patients with artificial pacemakers could have diagnostic implications in suggesting possible underlying complicating cardiac abnormalities.