Bicuspid aortic valve (BAV) is one of the more common congenital anomalies. It is well known that the ascending aorta and aortic root sometimes dilate in patients with BAV, even when the valve function is normal. We examined the morphological type and histological features of the dilated ascending aorta in patients with BAV. Of 276 patients who underwent aortic valve replacement (including coronary artery bypass grafting) from 2004 onwards, 60 (21.5%) with BAV were included in this study. The type of BAV was defined according to the Sievers classification. Type 1 BAV was the most common, and enlargement of the ascending aorta (≥45 mm) was the most common in the L/R type of BAV (48%). The morphology of the dilated ascending aortic wall was evaluated using three-dimensional CT angiography. The majority of dilations were asymmetric, but dilation was symmetric in the patient with dilation of the aortic root. Histological examination graded cystic medial necrosis of the ascending aortic walls using the aortic wall score. All patients with BAV had degeneration of the aortic wall, even when there was no dilation. The aortic walls of patients with dilated aortic roots showed advanced degeneration compared with the aortic walls of other patients. Therefore, aggressive root replacement may be appropriate, when the root is mildly dilated in patient with BAV.