AbstractIn 11 of 34 patients with typical mitral valve prolapse on the M‐mode echocardlogram, cross‐sectional echocardiography in the apical four‐chamber view revealed a characteristic abnormality not previously described. In mid‐ to late‐systole the ventricular septum contracted vigorously, forming a bend or sharp convexity toward the left ventricle, while motion of the posterolateral basal left ventricle was diminished and delayed. This finding was not encountered In 35 other patients with innocent systolic murmurs, or in 15 patients with mitral regurgitation of etiologies other than mitral valve prolapse. This abnormal pattern of left ventricular contraction on cross‐sectional echocardiography occurred with equal frequency In patients with holoaystollc and late‐systolic prolapse; it was seen more frequently In patients with severe prolapse than In those with mild to mode