Background: Post-stenotic aortic root dilation is often observed in aortic stenosis (AS) patients, but the relation to AS severity has not been reported from a large population of asymptomatic AS patients. Methods: Baseline data in 1223 patients with asymptomatic AS (mean age 67 years, 39% women) in the Simvastatin and Ezetimibe in Aortic Stenosis (SEAS) study was used. Annular diameter was measured from inner to inner edge in mid-systole. Inner and leading edge aortic root diameter was measured at the levels of sinus of Valsalva, sinotubular junction and supracoronary. Predicted aortic diameters were calculated from age and body size adjusted equations derived in normal adults. Results: On average, mean aortic annulus diameter was 2% larger, sinus of Valsalva diameter 11% larger, sinotubular junction diameter 22% larger and supracoronary diameter 32% larger in SEAS participants than that predicted from body size. Enlarged aortic diameter was found in 42%, 78%, 96% and 98% of patients at these levels and associated with larger inner diameter and thicker aortic wall (both p<0.01). In multivariate regression analysis, larger sinus of Valsalva or sinotubular junction diameter was independently associated with more severe AS (Table). Conclusions: Post-stenotic aortic root dilatation is common in asymptomatic AS with increasing prevalence from proximal to distal part. Aortic root dilation at the sinus of Valsalva and sinotubular junction level was associated with higher AS severity.