Cardiac Resynchronization Therapy. Background: A significant proportion of patients with dilated cardiomyopathy and left bundle branch block (LBBB) do not respond to cardiac resynchronization therapy (CRT). The purpose ofthis study was to investigate whether the electromechanical properties of the myocardium would predict acute hemodynamic improvement during left ventricular (LV) pacing. Methods and Results: We studied 10 patients with idiopathic dilated cardiomyopathy and LBBB (ejection fraction (EF): 27% ′ 7%; QRS duration: 166 ′ 16 msec) using three-dimensional electromechanical endocardial mapping technique to assess endocardial activation time (Endo-AT), unipolar voltage, and local linear shortening during sinus rhythm. LV stimulation was performed in VDD mode at live different sites and three atrioventricular delays within the coronary sinus. LV + dP/dt m a x changes from baseline were measured during LV stimulation at each site (%ΔdP/dt m a x ). There was no significant relationship between maximum % ΔdP/dt m a x during LV stimulation at the best coronary sinus site and LV EF, baseline LV + dP/dt m a x , total LV Endo-AT, baseline QRS duration nor changes in QRS duration during LV pacing. However, the maximum %ΔdP/dt m a x was significantly positively correlated with percentage area of late Endo-AT (r = 0.97, P 20% of LV area with late Endo-AT and >30% of preserved LV myocardium had five times better acute hemodynamic response with LV stimulation. Multivariate analysis showed that only percentage area of late Endo-AT was independently correlated with %ΔdP/dt m a x (P < 0.05). Conclusion: The presence of a larger amount of LV area with late Endo-AT and preserved LV myocardium measured by electromechanical mapping could identify patients who have better acute improvement in systolic performance during LV stimulation.