The purpose of this study was to investigate whether stimulation at the left ventricular (LV) septum (LVs), alone or in combination with another site, could be an alternative way to apply cardiac resynchronization therapy (CRT) that avoids the coronary sinus and phrenic nerve stimulation and may create more physiological sequence of activation.In CRT, biventricular pacing is commonly performed from the right ventricle (RV) and the epicardium of the LV lateral wall (LVlat). In the left bundle branch block (LBBB), half of the electrical delay occurs due to impulse conduction across the septum.Experiments were performed in 13 dogs with LBBB, 7 of them with chronic myocardial infarction (LBBB?+ MI). Pacing leads were positioned in the right atrium, RV, LVs, and at the LVlat epicardium. LV pump function was measured using conductance catheter and synchrony of electrical activation of the ventricles using epicardial mapping and from surface electrocardiogram. In 12 CRT patients, LV pump function was measured during temporary RV?+ LVs pacing and compared to RV?+ LVlat and RV?+ LVlat endo pacing.In the animals, electrical and hemodynamic benefits of LVs and RV?+ LVs pacing were comparable to those during?conventional biventricular pacing and were comparable in LBBB and LBBB?+ MI hearts. Dispersion of repolarization was reduced by LVs stimulation, but not by LVlat pacing. In patients, hemodynamic benefits of RV?+ LVs, RV?+ LVlat and RV?+?LVlat endo pacing were similar.The use of the LVs as LV pacing site in CRT improves synchronization and acute hemodynamics comparably to conventional biventricular pacing in dyssynchronous canines and in patients. In addition, LVs stimulation may reduce dispersion of repolarization compared to epicardial pacing. American College of Cardiology Foundation.