Purpose: Cardiac resynchronization therapy (CRT) has a positive effect on the improvement of functional mitral regurgitation in patients with heart failure with reduced ejection fraction. However geometric changes in the mitral valve apparatus, subvalvular structures and their contribution to the improvement of mitral regurgitation after CRT have not been clearly defined. The aim of our study was to evaluate the geometric parameters of mitral valve apparatus measured with 3Dimensional (3D) transesophageal echocardiography (TEE) before CRT implantation and to determine the parameters predicting the improvement of mitral regurgitation after CRT. Methods: In this prospective study thirty patients with moderate or severe mitral regurgitation with low EF heart failure planned for CRT implantation and had an indication for TEE were included. Effective regurgitant orifice (ERO) and regurgitant volume (RV) measurements were performed before CRT implantation. Detailed quantitative measurements of mitral valve were done from recorded images by 3D TEE. ERO, RV measurements were repeated to evaluate mitral regurgitation at the end of 3rd month. Results: There were no significant changes in left ventricular EF and left ventricular diameters at third month follow-up, whereas ERO and RV values were decreased. posterior leaflet angle was found higher in non-responder group compared to responder group. (28,93 ± 8,41 vs 41,25 ± 10,90, p = 0,006). Conclusion: Heart failure patients with moderate or severe functional mitral regurgitation who underwent CRT implantation were found lower posterior leaflet angle which was measured by 3D TEE in the patient group whose mitral regurgitation improved after CRT.