Background: Quadricuspid aortic valve is a rare congenital malformation of aortic valve. It has been an incidental finding at open heart surgery or at autopsy in the past. There has in recent years been a few case report before surgery, with the increasing use of echocardiography. In this study, we sought to characterize the clinical features of quadricuspid aortic valve (QAV) in our experience with 20 cases. Methods: The medical records of 103,106 patients who underwent echocardiography or aortic valve replacement from January 2000 to August 2011 in our institution were retrospectively analyzed. There were 11 men and 9 women with ages ranging from 37 to 79 years (60.1짹10.9 years). Results: The incidence of QAV was 0.019% in our institute. All the patients with QAV represented aortic regurgitation (AR) of various grade from mild to severe. While 5 patients had severe AR, only one had severe aortic stenosis with moderate AR among the 6 patients who received aortic valve replacement (AVR). The mean size of ascending aorta was 35.3짹4.3mm and 3 patients (15%) had dilated ascending aorta more than 40mm in the diameter. In classifying the Hurwitz���s type depending on the variation of cusp size, type A and C was most common (A: 6, B: 4, C: 6, D: 2, G: 2). Conclusion: The grade of AR was not associated with anatomical variation related to the cusp size. Unlike bicuspid AV, the cases of dilated ascending aorta were rare in the QAV patients and there was no significant increment of the ascending aortic diameter even in following AVR.