Mitral regurgitation (MR) is a pathological heart murmur which is the most likely murmur to be associated with reduced performance in performance horses. The mitral valve is the second most common location for acquired valvular disease in horses. MR increases the left-atrial pressures and left-atrial dilation which can predispose the horse to developing atrial fibrillation and pulmonary hypertension. It is also the most likely murmur to be associated with sudden death and congestive heart failure of the horse. MR can be caused by mitral valve dysplasia, degenerative or inflammatory valve thickening (including bacterial endocarditis), prolapse (MVP), thickened or ruptured chordae tendineae (RCT), and flail leaflet. MR also can develop secondary to valve annulus or ventricular dilatation (as with severe AR, nonrestrictive VSD or rarely, dilated cardiomyopathy). Clinical signs vary depending on the cause and degree of regurgitation. During the initial stages, MR may be detected as an incidental finding.