The mitral apparatus is a complex structure composed of several components, each of which can be affected by a variety of diseases, resulting in mitral regurgitation. The physiologic consequences of mitral regurgitation include reduced forward stroke volume; increased left atrial volume and pressure; and reduced resistance to left ventricular ejection. The latter explains why indices of systolic left ventricular function (ejection fraction) are often increased early in the course of mitral regurgitation. With the insidious development of mitral regurgitation, the left atrium dilates to accommodate the increase in volume, thereby reducing the atrial pressure. However, with the acute development of mitral regurgitation into a nondilated left atrium, pressure rises rapidly, producing pulmonary edema. The predominant clinical symptoms in chronic mitral regurgitation of dyspnea and fatigue result from pulmonary venous hypertension and low cardiac output. The cardinal physical finding is a mitral systolic murmur. Since the murmur can assume various configurations, the most reliable way to establish its correct origin is by bedside physiologic maneuvers. Typically, in the beat following a premature contraction or after a long pause during atrial fibrillation, the murmur of mitral regurgitation is unchanged in intensity, but murmurs due to left ventricular outflow obstruction increase. Also, isometric handgrip exercise increases the intensity of the murmur and a Valsalva maneuver decreases it during the strain phase. Echocardiography is the most useful noninvasive technique for evaluating patients with mitral regurgitation. Visualization of the mitral apparatus may establish the etiology of regurgitation, and measurement of left atrial size and left ventricular size and performance is useful for assessing the functional significance of the lesion. Doppler echocardiography can establish the diagnosis of mitral regurgitation in difficult cases with multi valve disease and can estimate the severity of the regurgitation. Cardiac catheterization and angiography are usually reserved for the patient being considered for valvular surgery. The natural history of chronic mitral regurgitation is characterized by slowly progressive symptoms, and often the onset of disabling symptoms is the result of irreversible left ventricular dysfunction. Medical therapy consists of digitalis, diuretics, and vasodilators for symptomatic patients. When symptoms occur despite this therapy, valvular surgery should be considered before left ventricular function becomes abnormal.