Background: Flail aortic valve(FAV) detected by echocardiography is an important cause of aortic regurgitation(AR). However, its unique clinical and pathologic features compared with those of other etiological diseases were not clearly elucidated. Methods: Among 392 patients with moderate to severe AR, 40 patients (10%) showed primary FAV without other etiology (Group A). We compared their clinical characteristics with rheumatic(Group B, n=117) or degenerative(Group C, n=46) cases. Aortic valve pathology was analyzed by four-scale scoring (0-3) using representative specimens of each groups. Results: Among Group A, right coronary cusp was most frequently involved(n=23, 58%), and 83% were NYHA class I or II at the first presentation. The mean age of Group A(53짹15 yrs) was significantly higher than that of Group B(46짹13 yrs, p<0.001), and lower than that of Group C(64짹8 yrs, p<0.001). The proportion of male sex in Group A(85%) was significantly larger than that of Group B(56%) or Group C(57%)(p<0.005). By pathologic analysis, aortic valve of Group A showed significantly larger amount of myxoid tissue than Group B(p<0.05) or Group C(p<0.05), but less amount of fibrosis than Group B(p<0.001) or Group C(p<0.05). The degree of calcification of Group C was higher than that of Group A(p<0.005) or Group B(p<0.05)(Table). Conclusion: Primary FAV without other causes occupies substantial proportion of significant AR, and it seems to be a unique disease entity different from rheumatic or degenerative aortic valve disease in clinical and histologic aspects.