Despite variable clinical results, beta blockers have become the primary therapy for prevention of aortic dilation in patients with the Marfan syndrome. This study examines the use of the angiotensin-converting enzyme inhibitor enalapril for treatment of these patients. We sought to examine the effects of enalapril versus beta-blocker therapy in patients with the Marfan syndrome and noted improved aortic distensibility (3.0 +/- 0.3 vs 1.9 +/- 0.4 cm2 dynes(-1); p <0.02) and a reduced aortic stiffness index (8.0 +/- 2.9 vs 18.4 +/- 3.8; p <0.05) in patients receiving enalapril compared with those receiving beta blockers. These favorable hemodynamic changes were associated with a smaller increase in aortic root diameter (0.1 +/- 1.0 vs 5.8 +/- 5.2 mm) and fewer clinical end points during follow-up.