Abstract: Bileaflet heart valves are illustrated which employ a pair of leaflets having flat inflow and outflow surfaces that are nonparallel to each other with the downstream arcuate edges being thinner than the mating upstream edges. The pivot mechanisms are constructed so that the leaflet inflow surfaces can be oriented parallel to the centerline through the valve body in the full open position, with the offset outflow surfaces constituting a diffuser section which provides for pressure recovery and reduces overall pressure drop through the valve. The pivot structures are preferably generally ellipsoidal in shape, with the long axis of the ellipsoid being aligned generally in an upstream-downstream direction. This feature together with the offset outflow surfaces of the leaflets promotes prompt closing of the valve.