Thirty-four patients with symptoms suggesting mitral valve prolapse were evaluated by M-mode echocardiography, real time, two-dimensional, phase array echocardiography, and cardiac catheterization. Ten of these patients had unequivocal evidence of mitral valve prolapse during cardiac catheterization. This diagnosis was established initially by M-mode and two-dimensional echocardiographic techniques. A characteristic "whiplike" motion of the posterior mitral leaflet was seen in 7 patients. The posterior mitral leaflet was moving successively toward the left atrium and left ventricle during the cardiac cycle. Arching of one or both mitral leaflets was also seen in patients with mitral valve prolapse during two-dimensional echocardiography. Two-dimensional echocardiography significantly enhances the mitral valve prolapse evaluation, but further experience and better techniques are necessary to explain many of the signs associated with this abnormality.