A valve prosthesis is sized and configured to rest within a blood path subject to antegrade and retrograde blood flow. A trestle element on the prosthesis extends across the blood path. A leaflet assembly is suspended from the trestle element and extends into the blood path in alignment with blood flow. At least one mobile leaflet member on the leaflet assembly is sized and configured to assume orientations that change according to blood flow direction. The mobile leaflet member has a first orientation that permits antegrade blood flow and a second orientation that resists retrograde blood flow. The valve prosthesis, when implanted in a heart chamber or great vessel, serves to supplement and/or repair and/or replace native one-way heart valve function.