The study objective was to compare coaptation, and leaflet and chordal stresses in normal and dilated mitral valves (18% annular dilatation) versus valves with flexible (Duran) and rigid (Carpentier-Edwards classic) ring annuloplasty, using a computer model. We have developed a 3D finite element model which allows us to evaluate valvular function in terms of coaptation and stresses in both leaflets and individual chordae.The mitral valve was simulated using ANSYS 4.4A software. Normal model geometry, collagen fiber orientation, tissue thickness and material properties were determined from fresh porcine valves. For annular dilatation, the annular circumference was increased by 18% versus normal. For annuloplasty, a simulated flexible ring was attached to the annulus, and a simulated rigid ring then attached. Valves were evaluated during systolic pressure loading, after which timing of coaptation and leaflet and chordal stresses were determined.In the normal valve, the anterior leaflet was subject to higher tensile stresses than the posterior leaflet which was under compression. With annular dilatation, all stresses were increased, particularly in the posterior leaflet. The flexible ring returned leaflet and chordal stresses closer to normal than did the rigid ring. Leaflet coaptation began at 5 ms in the normal state, was delayed by dilatation, and returned towards normal with both rings. The flexible ring returned coaptation and stresses closer to normal than did the rigid ring.Ring annuloplasty reduces the stresses and improves coaptation relative to annular dilatation. The success of mitral annuloplasty is likely due to the re-establishment of posterior leaflet compressive stresses and near-normal coaptation.