Background: Mitral regurgitation (MR) is frequent and its prevalence is increasing with aging of the population. Organic MR, due to primary valvular lesions has severe consequences determined by its degree, with left ventricular (LV) remodeling and dysfunction, left atrial (LA) enlargement, leading to poor clinical outcome. Surgery can eliminate MR, but carries notable risks and is not applicable to all patients. Recent animal data suggest that beta-blockade in organic MR has significant positive effect, particularly on LV remodeling. Therefore, chronically decreasing MR, protecting LV and LA with beta-blockade are major goals of medical therapy. However, effects of chronic oral beta-blockade of human MR are uncertain and recent practice guidelines underscored these gaps in knowledge and did not recommend beta-blockade of MR. Hence, a trial of treatment of organic MR is needed. A large trial with mortality-morbidity end-points is desirable but premature without knowledge of magnitude of mechanistic effects of beta-blockade. The improvement of these intermediate end-points, mechanistically linked to outcome, is measurable with non-invasive quantitative techniques and forms the basis of the present clinical trial proposal. Hypothesis: Chronic beta-blockade therapy using Metoprolol weighed against placebo produces a sustained reduction of the consequences of organic MR. Specific aims are that treatment improves a) degree of MR (decreases regurgitant volume, primary end-point), b) LV remodeling (decreases LV end-diastolic volume index, second end-point), and c) LA enlargement (decreases LA volume, third end-point) as compared to placebo. Population: Patients with MR organic (intrinsic valve disease), isolated (no other valve disease) d moderate (regurgitant volume *30 mL/beat). Methods: A randomized clinical trial, placebo controlled, double-blind, without crossover, of 12 months oral treatment with potent beta-blockade (Metoprolol XL 50 to 200mg QD) titrated to the maximally tolerated dose. The trial is preceded by an acute study to determine tolerance. End-points are measured by Doppler-Echocardiography for quantitation of MR (regurgitant volume) using combination of 3 simultaneous methods (quantitative Doppler, two-dimensional echocardiography, proximal flow convergence) and echocardiography for LV and LA volume measurement. In addition cardiopulmonary exercise testing will measure peak O2 consumption at baseline and follow-up. This study seeks to enroll a total of 60 patients. The analysis will be based on intention to treat and compare changes in regurgitant volume, LV end-diastolic volume index and LA volume measured after one year of treatment with active drug or placebo. The results of this clinical trial should provide strong evidence regarding medical treatment of patients with organic MR and define future strategies to minimize mortality and morbidity of organic MR.