Aneurysm of the mitral valve occurs most commonly in association with infective endocarditis of the aortic valve. The probable mechanism of its formation is destruction of the aortic valve which results in a regurgitant jet that strikes the anterior leaflet of the mitral valve, creating a secondary site of infection leading to the development of an aneurysm. Perforation of these aneurysms may occur, resulting in mitral regurgitation and pulmonary edema from a ventricle already volume overloaded from aortic regurgitation. This report describes the clinical and echocardiographic-pathologic findings in five patients with pathologically proven aneurysm of the mitral valve. There are no clinical features that appear specific for this abnormality. The two-dimensional echocardiographic feature that is helpful in the diagnosis is a bulge of the mitral valve leaflet toward the left atrium that persists throughout the cardiac cycle. Preoperative diagnosis is important because a mitral valve aneurysm may produce serious complications and is frequently overlooked during surgery. Repair of the aneurysm may be feasible; otherwise, valve replacement becomes necessary. Careful two-dimensional echocardiographic examination should be done in patients with left-sided infective endocarditis to detect an aneurysm of the mitral valve.