To study the effect of losartan (an angiotensin receptor blocker-ARB) on aortic root growth in patients with Marfan syndrome, already treated with beta-blockers (BB). The effect of losartan will be compared to placebo. Losartan or placebo will be added to the treatment regimen in a two-step up-titration scheme over 2 weeks. Start doses of Losartan will be 25 mg for subjects under 50kg of weight and 50mg if the weight is over 50kg. Uptitration will be guided by the tolerance of the drug by the patients. Patients will be contacted by phone call for assessment of side-effects before second step of uptitration.