Twenty-five patients, 16 men and 9 women, 5-59 years of age with the Marfan syndrome and cardiac complications were started on propranolol over a 6-year period. Propranolol was used as a beta-adrenergic blocking agent to reduce myocardial contractility in an attempt thereby to stay the progression of aortic dilatation and to prevent acute dissection of the aorta. The indications for prophylactic treatment were aortic dilatation, with aortic regurgitation in most cases. It was intended to keep the pulse rate below 70 at all times or below 60 at rest. Propranolol given in daily doses of 120-160 mg caused no side effects. The mean observation time for propranolol treatment in the 25 patients was 3.0 +/- 1.8 years ranging from 1-7 years. In spite of treatment, 5 patients (1 female and 4 males) experienced acute aortic dissection of rupture with fatal outcome. This occurred in 2 patients with the asthenic type, in 2 patients with the nonasthenic type, and in 1 patient with the marfanoid hypermobility syndrome. Serial echocardiograms showed that other patients on propranolol developed increasing dilatation of the aortic root. These observations indicate that propranolol does not necessarily protect against aortic dissection nor stop the progression of the aortic dilatation when cystic medial necrosis of the aorta is already present.