Mitral regurgitation is dysfunction of the mitral valve characterized by incomplete valve closure, resulting in systolic flow of blood from the left ventricle to the left atrium. The resultant increase in left atrial pressure often results in atrial fibrillation, dyspnea, and/or congestive heart failure. It is divided into 2 broad categories: primary (or degenerative) mitral regurgitation, which is an abnormality of the valve leaflets, and secondary (or functional) mitral regurgitation, which is incomplete leaflet closure secondary to disease of the left atrium or ventricle. Mechanisms of primary mitral regurgitation include leaflet prolapse, endocarditis, congenital anomalies, and rupture of chordae tendineae or a papillary muscle. Mechanisms of secondary mitral regurgitation include ischemia and dilated cardiomyopathy. Treatment and prognosis vary depending on the underlying cause.