Background: Although the geometric alterations determining the severity of functional mitral regurgitation (FMR) have been reported, 3-dimensional investigation into the main origin of FMR has not been demonstrated. Methods: Real-time 3-dimensional echocardiography (RT3DE) including 3D color Doppler imaging was performed in 54 patients with severe left ventricular (LV) dysfunction and sinus rhythm. With aliasing velocity set within 35 to 45 cm/s, the radius of proximal isovelocity surface area (PISA) was measured on the medial, central and lateral antero-posterior (A-P) planes perpendicular to commissure-commissure (C-C) plane at a mid-systole frame. The largest PISA radius among those 3 measurements was defined as PISAmax. The geometric analysis of LV and mitral valve apparatus was performed. At the same day, 2D Doppler echocardiography was also executed. Results: PISAmax was significantly correlated with the PISA radius measured in 2D color Doppler study (r=0.81, p<0.001). In 17 patients, PISA disappeared during the mid-systole phase. Among the rest, 22 patients (59%) showed PISAmax on the medial or lateral plane (Group A), while 15 patients (41%) on the central plane (Group B). In Group A, 13 patients (59%) showed dominant PISA in both commissural sides. Group A showed lower maximal velocity of FMR (p<0.05), larger LV spherical index on the C-C plane (p<0.01), larger angle between posterior leaflet and annular plane (p<0.05) and longer anterior leaflet distal to the angulation (ALd) on the central A-P plane (p<0.005) than Group B. The LV volume, ejection fraction, or the proportion of ischemic cardiomyopathy was not different between the two groups. ALd was the determinant of the eccentric dominated PISA by multiple logistic regression analysis (p<0.05), and ALd ��� 1.2 cm predicted it with sensitivity and specificity of 86% and 67%. Eleven patients (30%) with 2-3 separated PISAs presented smaller PISAmax than 26 patients with single PISA (3.2짹1.3 vs. 4.4짹1.6 mm, p<0.05). Conclusion: The dominant site of FMR is quite variable among patients with LV dysfunction. The eccentric origin of FMR might be dependent on the location of the secondary chordae causing anterior leaflet angulation.