Among 1519 patients undergoing diagnostic cardiac catheterization over a 2 yr 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. Of the patients 1/3 had associated coronary artery disease, aortic valve disease or atrial septal defects. Moderate or severe mitral regurgitation occurred in 14 (18%) patients. There was a wide spectrum of hemodynamic and ventriculographic abnormalities that were entirely explainable by the degree of prolapse, the severity of mitral regurgitation or the presence of associated diseases.