Echocardiography has been widely used for a detection of mitral and tricuspid valve prolapse. However, no extensive study has been made for a recognition of aortic valve prolapse. In this study, an echocardiographic search for aortic valve prolapse was attempted on 1700 consecutive cases at our laboratory including 70 cases with aortic regurgitation and 30 with mitral valve prolapse. Criteria of aortic valve prolapse by cross-sectional echocardiography were as follows: (1) eccentric coaptation point of aortic valve cusps, (2) asymmetric enlargement of the aortic valve cusp, and (3) downward displacement of the cusps. Using the above-mentioned criteria, five cases were recognized as having aortic valve prolapse, and it was divided into the following three morphological patterns: (1) prolapse of the right coronary cusp (1 case), (2) prolapse of the left coronary cusp (1) case, and (3) prolapse of the noncoronary cusp (3 cases). All 5 cases showed apparent aortic regurgitation, while none of these cases had any evidence of mitral valve prolapse. From the present observations, it might be suggestive that aortic valve prolapse is one of the common causes of aortic regurgitation. Elucidation of a spectrum of aortic valve prolapse should wait for a further extensive study.