A 32-year-old woman with Marfan's syndrome who had had a heart murmur in childhood was admitted due to congestive heart failure. Her echocardiography showed anterior and posterior leaflet prolapse of the mitral valve, and also severe mitral valve regurgitation. Her chest CT scan showed no evidence of an enlarged ascending aorta. We performed mitral valve replacement using a mechanical valve, because the long-term results of mitral valve repair for Marfan's syndrome are unknown. We reviewed the literature for other examples of this rare adult case with isolated mitral regurgitation associated with Marfan's syndrome.