BACKGROUND: Better prediction of cardiac pacing patients at risk of atrial fibrillation (AF) would enable more effective prophylaxis. We sought whether left atrial (LA) electromechanical conduction time (EMT) and myocardial mechanics were associated with incident AF in patients undergoing dual chamber pacemaker implantation, independent of left atrial volume (LAV). METHODS AND RESULTS: Clinical data were obtained prospectively in 146 enrollees (73±10 years) undergoing dual chamber pacemaker implantation in the Protect-Pace study. Echocardiograms and 2-dimensional strain analysis were obtained post implantation and at 2 years. Complete ascertainment of AF during follow-up was identified from interrogation of permanent pacemakers. Cox regression was used to identify correlates of AF. Incident AF (n=29, 20%) was associated with higher systolic blood pressure (P=0.01), lower left ventricular ejection fraction (P=0.03), lower LA strain at atrial contraction (LASac; P<0.001), higher LAV (P<0.003), and longer septal electromechanical conduction time (P<0.01). The associations of LAV and LASac with incident AF were independent of age, sex, systolic blood pressure, and left ventricular size and function. However, the combination of the 3 strongest predictors showed LASac (P=0.02) and systolic blood pressure (P=0.01) were independently associated with incident AF, but LAV was not (P=0.07). Using the optimal cut points from receiver operator characteristic curves (62 mL for LAV and 8.6% for LASac), we demonstrated that a significantly greater rate of AF was associated with both lower LASac at higher LAV and with lower LASac at lower LAV. CONCLUSIONS: The risk of AF in patients receiving dual chamber pacing is independently associated with LA size and function, not left ventricular structural and functional characteristics or right ventricular lead location. CLINICAL TRIAL REGISTRATION: URL: http://www.clinicaltrials.gov. Unique identifier: NCT00461734.