Echocardiographic examinations were performed in 26 patients with clinical evidence of Marfan's syndrome. Twelve patients were demonstrated to have isolated dysfunction of the mitral valvular apparatus of varying severity. Four patients demonstrated involvement of the aortic root as well as mitral valvular abnormalities, and six patients had problems involving the aortic root only. Four patients had no demonstrable cardiac abnormalities. Therapeutic decisions which could previously have been made with confidence only on the basis of cardiac catheterization with angiocardiographic studies were made by ultrasonic evaluation.