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Purpose: Cardiac resynchronization therapy (CRT) does not reduce the occurrence of atrial fibrillation (AF) in all responders. We investigated the factors associated with freedom from AF in patients after CRT.Method: Before and 6 months after CRT, mitral annular and inflow velocities, left atrial (LA) and LV volumes were measured, LA EF and emptying fraction were calculated. Regional systolic (Epsilon S) and late diastolic (Epsilon D) strain from the LA and right atrial (RA) walls were measured by speckle tracking. Intra-atrial delay was calculated as the standart deviation of LA regional time strain delays (Ts-SD) and inter-atrial delay as the time delay between the LA and RA free walls. Responders were defined with ≥15% reduction in LV end systolic volume.Results: Over 4 years of follow-up, 21(34%) out of 62 patients in sinus rhythm had persistent or permanent AF. AF incidence was lower in volume responders than in non-responders (21% versus 50%, P=0.015). Among the baseline variables Ts-SD and LA ejection fraction were independent predictors for AF. Changes in LA parameters, diastolic indices and LV volumes after CRT were significantly different between patients who developed AF and those who did not (Table; Δ = change). Increase in LA EF, decrease in LA volume index and E/Ea ratio were independent predictors of freedom from AF.Conclusions: LV volume response to CRT is enough to protect against AF, but improvement in LA function, LA reverse remodeling and a decrease in LA pressure as reflected by the change in E/Ea seem to be essential for freedom from AF after CRT.