Based on the observation that transforming growth factor beta (TGFbeta) plays an important role in the pathogenesis of aortic aneurysm formation in Marfan syndrome and that TGFbeta antagonism with losartan in Marfan mice significantly reduces aortic root growth, a placebo controlled study with losarten in humans will be conducted. Effect of treatment will be evaluated using vascular ultrasound, echocardiography with tissue Doppler imaging and magnetic resonance imaging.