Some authors recommend avoiding fusion with left ventricular (LV) intrinsic depolarization during cardiac resynchronization therapy (CRT). If fusion is still present during optimized biventricular (Biv) pacing and its long-term effects on the response to CRT are currently unknown. The aim of the study was to analyse the endocardial LV activation pattern induced by echocardiographically optimized Biv pacing and its influence on LV reverse remodelling.