Our criteria for the diagnosis of prolapsed mitral valve were proposed in this paper. With this abnormality, the principal alterations in the mitral valve consist of an abnormal systolic leaflet motion and mitral regurgitation with or without systolic clicks. Our criteria include: (1) auscultatory or phonocardiographic findings suggestive of mitral regurgitation or mitral complex abnormality, (2) a posterior displacement of the mitral valve throughout the latter half of systole by M-mode echocardiography, and (3) a systolic bulging or an apparent systolic ballooning of the mitral valve by two-dimensional echocardiography. A diagnosis of prolapsed mitral valve is justified in a given case if (1) plus one or two other findings are present. Several confusing M-mode and two-dimensional echocardiographic findings for the diagnosis of this condition are neglected in this criteria. We believe that the diagnosis of prolapsed mitral valve, as a rule, should depend on the presence of significant auscultatory or phonocardiographic finding which is a major manifestation of our criteria. However, our criteria seem to be inadequate for the diagnosis of a systolic ballooning of the mitral valve toward the left atrium which is frequently associated with atrial septal defect, since an apical systolic murmur is often absent. The mitral valve ballooning in atrial septal defect, however, is of functional origin, and should be differentiated from an organic prolapsed mitral valve. Thus, to be significant of prolapsed mitral valve, an apical auscultatory or phonocardiographic findings should be unequivocal.