AIM: Patients with obstructive sleep apnea (OSA) are likely to show increased arterial stiffness and progressive systemic atherosclerosis. Chest radiography reveals atherosclerotic changes in the aorta via measurement of aortic knob width. However, to our knowledge, aortic knob width in patients with OSA has never been evaluated. METHODS: We measured the aortic knob width in chest radiographs of 549 patients (age: 52.5±13.2 years; 69 women) who underwent overnight polysomnography. Moreover, we evaluated the association between aortic knob width and other clinical characteristics, including cardio-ankle vascular index (CAVI) and apnea-hypopnea index (AHI). Multivariate linear regression analysis was conducted to identify factors associated with aortic knob width. RESULTS: A significant direct correlation between aortic knob width and CAVI and between aortic knob width and AHI was observed. In multivariate linear regression analysis, either CAVI or AHI was independently associated with aortic knob width (p=0.004 and p＜0.001, respectively) in addition to age, male gender, body mass index, and systolic blood pressure. CONCLUSION: A significant independent correlation between aortic knob width and OSA severity was observed. Our findings suggest that an increase in the aortic knob width suggests atherosclerotic changes in the aorta and may be associated with OSA and increased arterial stiffness.