Objective: Patients with severe mitral regurgitation commonly develop atrial fibrillation. The precise mechanisms of this relationship remain unknown. The objective of this study was to apply noninvasive electrocardiographic imagingof the atria during sinus rhythm to identify changes in atrial electrophysiology that may contribute to development of atrial fibrillation in patients with severe mitral regurgitation referred for mitral valve surgery. Methods: Twentysubjects (9 atrial fibrillation andmitral regurgitation, 11 mitral regurgitation alone) underwent electrocardiographic imaging.Biatrial electrophysiology was imaged with activation maps in sinus rhythm. The reconstructed unipolar electrograms were analyzed for voltage amplitude, number of deflections and conduction heterogeneity. In subjects with mitral regurgitation, left atrial biopsies were obtained at the time of surgery.Results: Subjects with history of atrial fibrillation demonstrated prolonged left atrial conduction times (110±25 ms vs.mitral regurgitation alone (85±21), p=0.025); right atrial conduction times were unaffected. Variable patterns of conduction slowing were imaged in the left atria of most subjects, but those with prior history of atrial fibrillationhad more complex patterns of conduction slowing orunidirectional block. The presence of atrial fibrillation was not associated with the extent of fibrosis in atrial biopsies. Conclusions: Detailed changes in sinus rhythm atrial electrophysiology can be imaged noninvasively and can be used to assess the impact and evolution of atrial fibrillationon atrial conduction properties in patients with mitral regurgitation.If replicated in larger studies, electrocardiographic imaging may identifypatients with mitral regurgitation at risk for atrial fibrillation and could be used to guide treatment strategies.