Obstructive sleep apnea (OSA) is associated with an increased prevalence of aortic aneurysms, and it has also been suggested that severe OSA furthers aneurysm expansion in the abdomen. We evaluated whether OSA is a risk factor for the progression of ascending thoracic aortic aneurysms (TAA).Patients with TAA underwent yearly standardised echocardiographic measurements of the ascending aorta over 3 years, and two level-III sleep studies. The primary outcome was the expansion rate of TAA in relation to the apnea-hypopnea-index (AHI). Secondary outcomes included surveillance for aortic events (composite endpoints of rupture, dissection, elective surgery, and death).Between July 2014 and March 2020, 230 patients (median age 70 years, 78% male) participated in the cohort. At baseline, 34.8% of patients had an AHI of ≥15 events·h-1. There was no association between TAA diameters and the AHI at baseline. After 3 years mean expansion rates were 0.55±1.25 mm at the aortic sinus and 0.60±1.12 mm at the ascending aorta. In the regression analysis, after controlling for baseline diameter and cardiovascular risk factors, there was strong evidence for a positive association of TAA expansion with AHI (aortic sinus estimate 0.025 mm [95%CI 0.009 to 0.040], p<0.001; ascending aorta estimate 0.026 mm [95%CI 0.011 to 0.041], p=0.001). Twenty participants (8%) experienced an aortic event, however, there was no association with OSA severity.OSA may be a modest but independent risk factor for faster TAA expansion and thus potentially contributes to life-threatening complications in aortic disease.