Objective: The most significant complications seen in patients with obstructive sleep apnea syndrome (OSAS) are associated with the cardiovascular system. The present study assessed aortic stiffness in patients with OSAS and evaluated the effect of continuous positive airway pressure (CPAP) therapy on aortic stiffness. Method: Twenty-four patients with newly diagnosed, previously untreated, moderate or severe OSAS (apnea-hypopnea index > 15) and a control group of 17 healthy patients were included in the study. M-mode recordings of the ascending aorta were taken from the parasternal long axis by echocardiograhy, and systolic and diastolic diameters of the aorta were measured. Aortic elastic parameters, aortic strain, and distensibility were calculated. Measurements were repeated after 6 months of CPAP therapy in patients with OSAS and were compared with baseline values. Results: In patients with OSAS, compared with the control group, aortic strain (6.7% +/- 2.1% vs. 12.4% +/- 3.1%; P < 0.001) and aortic distensibility (2.8 +/- 0.9 x 10(-6) cm(2) dyn(-)1 vs. 5.5 +/- 1.7 x 10(-6) cm(2) dyn(-1); P < 0.001) were evidently lower, and there was a significant correlation between aortic elastic parameters and AHI. After a 6-month course of CPAP therapy, significant increases were observed in aortic strain (6.1% +/- 1.5% vs. 7.3% +/- 1.7%; P < 0.001) and aortic distensibility (2.5 +/- 0.7 x 10(-6) cm(2) dyn(-1) vs. 3.1 +/- 0.9 x 10(-6) cm(2) dyn(-1); P < 0.001) in patients with OSAS. Conclusion: Aortic strain and distensibility were lower in patients with OSAS than in control patients, and CPAP treatment provided improvement in aortic elastic parameters. (ECHOCARDIOGRAPHY, Volume 26, November 2009).