Cardiovascular complications are the leading cause of mortality and morbidity in Marfan syndrome (MFS), a dominantly inherited disorder caused by mutations in the gene that encodes fibrillin-1. There are approximately 18,000 patients in the UK with MFS. Current treatment includes careful follow-up, beta blockers, and prophylactic surgical intervention; however, there is no known treatment which effectively prevents the rate of aortic dilatation in MFS. Preclinical, neonatal, and pediatric studies have indicated that angiotensin receptor blockers (ARBs) may reduce the rate of aortic dilatation. This trial will investigate the effects of irbesartan on aortic dilatation in Marfan syndrome.