To assess the contributions of mitral leaflet billowing and exaggerated systolic mitral anular expansion to posterior motion of mitral leaflets recognized as mitral valve prolapse (MVP) by M-mode echocardiography, time-motion reconstructions of the anteroposterior displacement of points equally spaced along the anterior and posterior mitral leaflets were derived by computer-assisted analysis of 2-dimensional echocardiograms. Late or holosystolic posterior displacement of mitral leaflets, greater than or equal to 2 mm, occurred in the reconstructions from 24 of 24 (100%) patients with MVP with leaflet billowing and in 20 of 24 (83%) patients with MVP without leaflet billowing compared with 4 of 35 (11%) age-sex matched normal adults (both p less than 0.0000002). Posterior motion of the posterior mitral leaflet in time-motion reconstruction was significantly less with respect to the posterior end of the mitral anulus than with respect to the chest wall in patients with nonbillowing MVP (1.6 +/- 1.9 vs 2.7 +/- 1.6, p less than 0.02), but not in those with leaflet billowing (3.6 +/- 1.8 vs 3.9 +/- 1.8, p = not significant), because anular expansion contributed importantly to MVP in the former but not in the latter group. Thus, M-mode echocardiographic patterns of MVP reflect the separate but interacting effects of distinct abnormalities of mitral anular and leaflet dynamics.