The objective of the proposed research is to conduct a randomized, double-blind superiority feasibility study to compare the effects of two types of medication (Losartan, an Angiotensin Receptor Blocker, and Atenolol, a beta blocker) on vascular function in patients with Marfan syndrome (MFS). Our outcomes will include measuring the pulse wave velocity (PWV) and other biophysical properties of the aorta in the two groups of patients. This study will provide essential pilot data and development of logistics to enable a full multi-centre randomized controlled trial to test the hypothesis that Losartan will improve vascular function and, in the long-term, slow the rate of aortic root dilation.