Obstructive sleep apnea (OSA), characterized by partial or complete occlusion of the pharynx during sleep, results in persistent inspiratory effort and interruption of airflow. During each episode of apnea, OSA patients develop increased transmural pressure in the aortic wall. Accordingly, to test the hypothesis that the presence of OSA would be associated with greater thoracic aortic size, we prospectively assessed 150 consecutive patients, newly referred to the sleep clinic in our institution, in a crosssectional study to confirm OSA. The patients underwent sleep study and chest computed tomography (CT)-derived thoracic aortic diameter. In this particular period, a chest CT was performed within 3 months of the sleep study upon informed consent to our protocol. Exclusion criteria included: 1) prior history of aortic dissection, aortic valvular disease, and clinical characteristics of Marfan's syndrome; 2) central sleep apnea; 3) treatment for sleep apnea; and 4) dialysis. The outer diameter of the ascending aorta was measured by caliper within the CT image. Overnight sleep study was performed using cardiopulmonary monitoring (Morpheus, Teijin Inc., Tokyo, Japan). The apnea-hypopnea index (AHI) was quantified as the frequency of apneas and hypopneas per hour of bed time. OSA was defined as AHI Ն10/h. The data are presented as mean Ϯ SD or frequencies. To determine the independent factors, the multiple linear regression model with backward elimination technique was used, including older age, male gender, blood pressure, hypertension, dyslipidemia, diabetes mellitus, ischemic heart disease, smoking, and AHI. Comparisons between the 2 groups were performed by Student t test for the unadjusted aortic diameter and by analysis of covariance for the adjusted aortic diameter.The patients' mean age was 60 Ϯ 11 years; 125 (83%) were men, with mean body mass index of 24.7 Ϯ 3.1 kg/m 2 . Fifty-nine percent of patients had hypertension. At assessment, 91% of patients had already taken prescribed antihypertensive medications. One hundred ten patients (73%) had OSA. On univariate analysis, older age, male gender, body mass index, systolic blood pressure, pulse pressure, hypertension, dyslipidemia, ischemic heart disease, and AHI were positively correlated with thoracic aortic size. On multivariate analysis, older age (per 10-year increase, coefficient 1.82, 95% confidence interval [CI]: 1.13 to 2.34, p Ͻ 0.001), male gender (coefficient 3.25, 95% CI: 1.63 to 4.87, p Ͻ 0.001), and AHI (per 10-event/h increase, coefficient 0.62, 95% CI: 0.25 to 0.98, p Ͻ 0.001) remained as factors associated with greater thoracic aortic diameter. Contrarily, there was no significant independent relationship between blood pressure/hypertension and thoracic aortic size. Additionally, patients with OSA had a significantly greater thoracic aortic size than those without OSA (p Ͻ 0.001) (Fig. 1), even if adjusted for older age and male gender (p Ͻ 0.001).Increased thoracic aortic size is known to be related to aging, male, genetic mutation including M...