Among 1,519 patients undergoing diagnostic cardiac catheterization over a 2-year period, angiographically unequivocal mitral valve prolapse was identified in 79 cases (5.2%). Mitral valve prolapse was clinically unsuspected in 30 of these 79 patients. Echocardiographic studies were available in 44 cases, but only 55% of the patients with prolapse of the mitral valve documented by angiography had positive echocardiographic findings. One third of the patients had associated coronary artery disease, aortic valve disease or atrial septal defects. 14 patients (18%) had moderate or severe mitral regurgitation. There was a wide spectrum of hemodynamic and ventriculographic abnormalities that could not entirely be explained by the degree of prolapse, by the severity of mitral regurgitation or by the presence of associated diseases.