Background : FMR (functional mitral regurgitation) is a mechanical phenomenon that occurs with structurally normal valve leaflets. It develops in approximately 20% of the patients following myocardial infarction and 50% of those with congestive heart failure and even mild degree of FMR affects the prognosis. Therefore, the use of multiple echocardiographic parameters to assess FMR severity and its major mechanism in relation to the etiology is crucial for planning therapeutic options. Aim of the study : To determine the different mechanisms of functional mitral regurgitation in patients with left ventricular systolic dysfunction , correlate it to severity of mitral regurgitation and if possible to differentiate ischemic and non ischemic left ventricular dysfunction based on the predominant mechanism of FMR in each study group. Methods : 40 patients with functional mitral regurgitation and left ventricular systolic dysfunction (EF< 45%) who met the inclusion criteria were included. The study population was divided into two groups (based on coronary angiography ), group 1 (21 patients); patients with FMR due to ischemic cardiomyopathy and group 2 (19 patients); patients with FMR due to idiopathic dilated cardiomyopathy. All the studied population underwent history taking, clinical examination and ECG. Routine transthoracic echocardiogram (TTE) was done to evaluate mitral valve deformation indices, left ventricular remodeling parameters, left ventricular systolic performance indices including left ventricular global strain by 2D speckle tracking.Results : Mitral valve deformation indices: coaptation distance, MAA, tenting area and tethering distance of the posterolatreal papillary muscle were positively correlated with severity of mitral regurgitation in both study groups. Left ventricular remodeling specially mediolateral diameter were positively correlated with FMR severity in group 2 patients , GLS was positively correlated with FMR severity in both study groups and an important determinant of MR severity in group 2 patients.PM tethering distance was an important determinant of MR severity in both study groups . Severity of FMR tends to be increased in dilated cardiomyopathy patients. Conclusion: FMR is a mechanical phenomenon that occurs due to potential changes that affect mitral valve function. Augmented leaflets tethering due outward and /or apical displacement of the PMs with LV remodeling appears to be a basic mechanism and final common pathway for the genesis of FMR. Annular dilatation and LV dysfunction had an important contribution (acting as modulating factors that are able to increase significantly the severity of mitral regurgitation) in the presence of augmented leaflets tethering especially in cases of dilated cardiomyopathy. Based on disease categorization severe mitral regurgitation occurs more frequently in dilated cardiomyopathy patients that may be related to global LV remodeling associated with increased mitral annular area, tenting area and coaptation distance.