Simultaneous stimulation of 2 left ventricular (LV) sites could enhance the effectiveness of cardiac resynchronization therapy (CRT). The aim of this study was to evaluate the acute hemodynamic response to dual-site LV CRT. Two LV pacing leads were successfully implanted in 12 CRT candidates (New York Heart Association classes III to IV, QRS >or=120 ms). Target positions were the lateral or posterolateral vein (site A) and anterior or anterolateral vein (site B). A conductance catheter was placed in the left ventricle for pressure-volume measurements. Tested CRT configurations were alternated by atrial overdrive pacing at a fixed rate and included site A and B single-site CRT and dual-site LV CRT (2 LV sites plus right ventricular apex) at 4 atrioventricular intervals. Overall, single-site LV CRT significantly enhanced stroke volume, stroke work, maximum pressure derivative, and conductance-derived indexes of LV synchrony when delivered in site A, whereas no significant changes were noticed with pacing in site B. Specifically, site-A pacing resulted in a higher stroke volume increase (LV pacing site associated with the best hemodynamic response [best-LV]) in 8 patients, and site-B pacing, in 4 patients. At intermediate atrioventricular intervals, dual-site LV CRT resulted in improved stroke volume, stroke work, maximum pressure derivative, and LV synchrony with respect to single-site CRT when delivered at the best-LV (all p <0.05). However, single-site CRT at best-LV produced results similar to dual-site LV CRT when the atrioventricular interval was optimized in each patient. In conclusion, adding a second LV lead does not result in further improvement in acute hemodynamic response with respect to standard CRT when the single LV pacing site and atrioventricular interval are optimal.