Aim: To assess geometrical difference of the mitral apparatus in patients with functional mitral regurgitation (MR) according to MR severity using real-time 3D echocardiography (RT3DE) Methods: Fourteen patients with functional MR (ejection fraction: 25짹5 %, regurgitant orifice area: 0.24짹0.22 cm2) and 5 normal controls underwent RT3DE. Antero-posterior (AP) and commissure-commissure (CC) dimensions of the mitral annulus, tethering angle of anterior (A慣) and posterior leaflet (P慣) and mitral valve tenting area (MVTa) were measured in two orthogonal apical planes (CC plane, AP plane) with 3D computer software (4D Cardio-View, TomTec). For the estimation of the degree of papillary muscle (PM) displacement, the distances from the medial junction of mitral and aortic annulus (MMA) to each head of anterior and posterior PM distributing chordae to the anterior mitral leaflet were measured (Figure). Sum of these two distances (誇MMA-PM) was then calculated. Distance between two heads (dPM) was also measured. Sphericity of the LV chamber was estimated by the ratio between LV width at the level of the PM and the length from the annulus to the LV apex. Results: Among all measurements, MVTa, P慣, 誇MMA-PM and dPM showed significant correlations (p<0.05) with MR severity. As we explored the main determinants using multivariate stepwise linear regression analysis with those measurements showing significant correlations with MR severity, MVTa was found to be the strongest determinant of MR severity (R2= 0.79, p=0.04). Conclusion: Among all geometric measurements of the mitral apparatus, MV tenting area seems to be the strongest geometric factor determining the severity of functional MR. RT3DE is a useful tool to elaborate the geometry of the mitral apparatus including the specified PM head.