The purpose of our study was (1) to assess retrospectively, in healthy subjects and in patients with moderate and severe functional mitral regurgitation (FMR), the normal mitral annular dimensions, (2) to determine differences in mitral annular geometry between healthy subjects and patients with FMR, and (3) to evaluate potential errors in 2-dimensional (2D) measurements given the 3D nature of the mitral annulus. 15 patients with no cardiac abnormalities (referred to as normals), 13 with moderate and 15 with severe FMR as determined by echocardiography underwent contrast-enhanced cardiac 64-slice Computed tomography (CT) with prospective electrocardiography-gating for excluding coronary artery disease. With an advanced visualization, segmentation, and image analysis software, the area, intercommissural distance (CC), septolateral distance (SLD), and the anterior and posterior circumference of the MA were measured in diastole. We found significant (P < .001) differences between normals and patients with severe FMR for area, SLD and posterior circumference in 3D (P < .001) and 2D (P < .001). Similarly, the SLD and the posterior circumference in both 3D (P = .002) and 2D (P = .001) were significantly smaller in patients with moderate FMR as compared to those with severe FMR. In contrast, there were no significant differences between groups regarding the CC and the anterior circumference both in 3D and 2D (all, P > .05). Measurements in 3D differed significantly from those with 2D for all circumference measurements and groups (P < .01), with a systematic underestimation of the posterior circumference of 2.1 ± 1.5 mm in normals, 1.8 ± 1.3 mm in patients with moderate FMR, and 1.9 ± 1.9 mm in patients with severe FMR for 2D. Our study provides in vivo human CT data on MA dimensions in normals and patients with FMR, indicating differences in patients for the area, posterior circumference and SLD but not for the anterior circumference and CC. Systematic differences exist between 2D and 3D measurements for all circumferential measurements.