 Background: Cardiac resynchronization therapy CRT in patients in atrial fibrillation AF seems to provide modest favorable effect compared with RV pacing. In the Optimal Pacing SITE OPSITE trial the results were heterogeneous. The situation was almost certainly one in which some patients are showing marked clinical benefit with LV-based pacing, balanced by other patients in whom RV was the best choice. Therefore with the actual standards, CRT pacing cannot be recommended as a first line treatment for all patients with AF and refractory heart failure and new criteria to identify the patients who might benefit are needed. Furthermore, a pre-requisite for CRT in patients with AF is the presence of stable 3rd degree AV block which usually implies the need for performing catheter ablation of the AV junction. 