Background: Mitral valve (MV) tethering through basal chordae (BC) is considered as a pathogenic mechanism of functional mitral regurgitation (FMR). However, individual variation of BC insertion sites and its association with FMR have not been demonstrated. Methods: In 13 patients in sinus rhythm who underwent cardiac transplantation due to dilated (n=10) or ischemic (n=3) cardiomyopathy, 3-dimensional (3D) full-volume/color Doppler echo was performed before transplantation to analyze the geometry of MV apparatus, left ventricular (LV) volumes, and FMR severity using 3D proximal isovelocity surface area. After transplantation, 4 BC lengths and insertion sites were evaluated using direct measurements in the explanted hearts before fixation. Results: BC insertion sites on the MV were quite variable among patients. The mean longitudinal distance from the MV edge of inner 2 BC correlated significantly with distal length of anterior MV from the angulation point on the central anteroposterior plane of 3D echo (r=0.652, p=0.016). The square root value of effective regurgitant orifice area (EROA) correlated significantly with mean longitudinal distance from the MV edge of outer 2 BC (r=0.673, p=0.012) which also tended to correlate with tenting area (r=0.546, p=0.054). EROA or square root of EROA did not correlate with LV volume indices, LV ejection fraction, BC lengths and annulus diameters. By multiple linear regression analysis, mean longitudinal distance from the MV edge of outer 2 BC was the only determinant of EROA (p=0.045). The patients with EROA ��� 0.1 cm² had larger mean longitudinal distance from the MV edge of outer 2 BC than those with EROA < 0.1 cm² (1.1 짹 0.2 vs. 0.7 짹 0.3 cm, p=0.029). Conclusions: High individual variations exist in BC insertion sites on the MV. The inner BC insertion sites determine anterior MV central angulation point, and the outer BC insertion sites seem to be a major determinant of FMR severity in patients with dilated LV and severe systolic dysfunction.