The left ventricular cineangiograms of 22 asymptomatic, apparently healthy male aviators without noninvasive (echocardiographic or auscultatory) evidence of mitral valve prolapse were reviewed and compared with those of 12 men with noninvasive evidence of mild mitral valve prolapse. The maximal protrusion of the posterior mitral valve leaflet superior and posterior to a line perpendicular to the long axis of the left ventricle at end-systole was measured from the right anterior oblique left ventricular cineangiogram by repeated observation of left ventricular inflow. The values were 7.5 +/- 1.6 mm in patients without mitral valve prolapse and 11.2 +/- 3.4 mm in patients with mitral valve prolapse (mean +/- 1 standard deviation). This measurement did not exceed 11 mm in any patient without prolapse. It is concluded that: 1) with meticulous attention to angiographic landmarks of the left ventricular inflow area, the limits of normal systolic posterior mitral leaflet motion can be defined; and 2) systolic motion outside these limits constitutes a quantitative criterion for the angiographic diagnosis of mitral valve prolapse.