This trial is a randomized, double-blind trial of 50 adults with Marfan syndrome, treated with 6 months of atenolol vs. losartan. Arterial tonometry for aortic stiffness and echocardiography for diastolic function will be performed at the beginning and end of treatment. A blood draw for serum markers of extracellular matrix turnover and inflammation will also be performed at 0 and 6 months. We plan to determine whether losartan decreases aortic stiffness and left ventricular diastolic dysfunction significantly more than atenolol.