Background: Functional mitral regurgitation (FMR) occurs as a consequence of left ventricular remodeling and is an independent predictor of adverse outcome. FMR is assessed qualitatively with two-dimensional echocardiography, but accurate quantitation of the actual degree of mitral valve (MV) coaptation is not possible with this method. We evaluated a novel three-dimensional (3D) approach to quantify the MV coaptation zone in patients with FMR. We hypothesized that measuring the 3D MV coaptation zone is feasible and would correlate with FMR severity when indexed to MV area., Methods: Data were gathered on 25 patients with FMR undergoing cardiac operations, and included a comprehensive two-dimensional and 3D examination with intraoperative transesophageal echocardiography. Using available 3D MV quantification software, offline analysis of end-systolic MV coaptation zone and MV area was performed. A novel MV coaptation index was calculated by the following formula: [3D end-systolic MV coaptation zone/3D MV area]. FMR severity was described as trace, mild, moderate, and severe using the integrative approach recommended by official guidelines., Results: Analysis of variance demonstrated that the coaptation index was associated with the severity of FMR (F = 20.5, r(2) = 0.75, p < 0.0001). There was also a correlation between 2D vena contracta and the coaptation index (r = -0.74, p < 0.0003)., Conclusions: We describe a novel 3D approach to direct assessment of the MV coaptation zone. When indexed to the MV area, the 3D MV coaptation zone is closely associated with FMR severity. Assessment of the mitral coaptation may be a potentially powerful tool in the perioperative evaluation of the competency of the MV., (Copyright © 2014 The Society of Thoracic Surgeons. Published by Elsevier Inc. All rights reserved.)