Comparison of Two Medications Aimed at Slowing Aortic Root Enlargement in Individuals With Marfan Syndrome

Inclusion Criteria: * Diagnosis of Marfan syndrome, according to Ghent criteria (more information can be found in Appendix D of the protocol) * Aortic root Z-score greater than 3.0 Exclusion Criteria: * Prior aortic surgery * Aortic root dimension at the sinuses of Valsalva greater than 5 cm * Planned aortic surgery within 6 months of study entry * Aortic dissection * Shprintzen-Goldberg syndrome * Loeys-Dietz syndrome * Therapeutic (i.e., for arrhythmia, ventricular dysfunction, or valve regurgitation) rather than prophylactic use of angiotensin-converting enzyme (ACE) inhibitor, beta-blocker, or calcium channel blocker * History of angioedema while taking an ACE inhibitor or beta-blocker * Intolerance to losartan or other angiotensin II receptor blocker (ARB) that resulted in termination of therapy * Intolerance to atenolol or other beta-blocker that resulted in termination of therapy * Kidney dysfunction (i.e., creatinine greater than the upper limit of age-related normal values) * Asthma of sufficient severity to prohibit the use of a beta-blocker * Chronic use of steroids and/or beta-adrenergic agents with exacerbations of asthma that are frequent (averaging three or more per year) or severe (requiring hospitalization) * Diabetes mellitus * Pregnant or planning to become pregnant within 36 months of study entry * Inability to complete study procedures, including history of poor acoustic windows (i.e., inability to obtain accurate measurement of aortic root)