Background: Patients with atrial fibrillation (AF) and left atrial (LA) enlargement may develop functional, normal leaflet motion mitral regurgitation (MR) without left ventricular (LV) remodeling. Mitral annular dynamics and LV mechanics are important for preserving normal mitral valve function. The aim of this study was to assess the annular and LV dynamics in patients with AF and functional MR. Methods: Twenty-one patients with AF with moderate or more MR (AFMR+group), 46 matched patients with AF with no or mild MR (AFMR- group), and 19 normal patients were retrospectively studied. Mitral annular dynamics were quantitatively assessed using three-dimensional echocardiography. Systolic LV global longitudinal strain (GLS), global circumferential strain, and LA strain were measured using two-dimensional speckle-tracking echocardiography. Results: The normal annulus displayed presystolic followed by systolic contraction and increase in saddle shape (P < .01 for all). Presystolic annular dynamics were abolished in both groups of patients with AF (P > .05 vs normal). In contrast, systolic and total annular dynamics during the cardiac cycle were preserved in AFMR- patients (P > .10 vs normal) but impaired in AFMR+patients (P < .05 vs normal and AFMR-). LV GLS (P < .0001) and LA strain (P = .02), but not LV global circumferential strain (P = .97), were impaired in AFMR+compared with AFMR- patients despite comparable LA and LV volumes. MR severity correlated with systolic annular contraction (r = 0.64, P < .0001), saddle deepening (r = 0.53, P = .003), and LV GLS (r = 0.46, P < .0001). Multivariate analysis identified that impaired systolic contraction (odds ratio, 2.18; P = .001) and saddle deepening (odds ratio, 2.68; P = .04) were independently associated with MR. Excluding annular dynamics from the model, less negative LV GLS, but not LA strain, became associated with MR (odds ratio, 1.93; P < .0001). Conclusions: In patients with AF and absent LA contraction, the normal predominantly "atriogenic'' annular dynamics become "ventriculogenic.'' Isolated LA enlargement is insufficient to cause important MR without coexisting abnormal LV mechanics and annular dynamics during systole. "Atrial'' functional MR may not be purely an atrial disorder.