This trial is a randomized, open-label trial of 32 patients with Marfan syndrome, treated with 6 months of aliskiren vs. negative controls in patients with MFS under atenolol treatment. MRI for aortic pulsed wave velocity (PWV) and distensibility, measurements of central BP (CBP) and augmentation index (AIx) will be performed at the beginning and end of treatment. A blood drawn for serum markers of TGF-beta, extracellular matrix turnover and inflammation will also be performed at 0 and 6 months. We plan to determine whether aliskiren decreases aortic stiffness significantly more than negative controls in patients with MFS under atenolol treatment.