Mitral valve prolapse is a pathologic anatomic and physiologic abnormality of the mitral valve apparatus affecting mitral leaflet motion. "Mitral valve prolapse syndrome" is a term often used to describe a constellation of mitral valve prolapse and associated symptoms or other physical abnormalities such as autonomic dysfunction, palpitations and pectus excavatum. The importance of recognizing that mitral valve prolapse may occur as an isolated disorder or with other coincident findings has led to the use of both terms. Mitral valve prolapse syndrome, which occurs in 3 to 6 percent of Americans, is caused by a systolic billowing of one or both mitral leaflets into the left atrium, with or without mitral regurgitation. It is often discovered during routine cardiac auscultation or when echocardiography is performed for another reason. Most patients with mitral valve prolapse are asymptomatic. Those who have symptoms commonly report chest discomfort, anxiety, fatigue and dyspnea, but whether these are actually due to mitral valve prolapse is not certain. The principal physical finding is a midsystolic click, which frequently is followed by a late systolic murmur. Although echocardiography is the most useful mode for identifying mitral valve prolapse, it is not recommended as a screening tool for mitral valve prolapse in patients who have no systolic click or murmur on careful auscultation. Mitral valve prolapse has a benign prognosis and a complication rate of 2 percent per year. The progression of mitral regurgitation may cause dilation of the left-sided heart chambers. Infective endocarditis is a potential complication. Patients with mitral valve prolapse syndrome who have murmurs and/or thickened redundant leaflets seen on echocardiography should receive antibiotic prophylaxis against endocarditis.