A defect in the MITRAL VALVE of the HEART which allows blood to leak from the left VENTRICLE into the left ATRIUM. It is also known as mitral regurgitation; incompetence may occur along with MITRAL STENOSIS. The left ventricle has to work harder to compensate for the faulty valve; it enlarges, but eventually the ventricle cannot cope with the extra load and left-sided heart failure may develop. A common cause of mitral incompetence is RHEUMATIC FEVER or damage following a heart attack. The condition is treated with drugs to help the heart, but in severe cases heart surgery may be required.