Marfan syndrome is a genetic disease of our connective tissue, which provides material and support for our skeleton, muscles, blood vessels and other parts of our bodies. People with Marfan syndrome may be tall and thin with slender, tapering fingers, long arms and legs, and spine curvature. They often have heart and eye problems. In some patients, the condition is very mild and the person has few or no symptoms. Others are always at risk of life-threatening problems, which usually involve damage to the valves in the heart or weakening of the large blood vessels leading from the heart. If the blood vessels become weak, they can balloon out (dilate) and break (rupture), which might cause the person to die suddenly. We have only a limited ability to stop the progression of disease in Marfan syndrome. Typically we use medicines that lower heart rate or blood pressure (or both). But this does not prevent the disease and very few drugs work well enough to keep patients from needing surgery or dying suddenly because a blood vessel has torn open. Our objective is to study two medicines to see if one, or both, can improve blood vessel function in patients with Marfan syndrome. One (Atenolol) belongs to a group of drugs called beta blockers and is often used to treat high blood pressure. It is the most common drug that is currently used to treat patients with Marfan syndrome. The other (Losartan) is also used for high blood pressure, but works in a different way. This study will help us to find better ways to treat people who have Marfan syndrome and to identify early changes in blood vessel function that may help to prevent long-term complications.