In one embodiment, the present invention provides a prosthesis that can be implanted within a heart to at least partially block gaps that may be present between the two mitral valve leaflets. In one preferred embodiment, the prosthesis includes an anchoring ring that expands within the left atrium to anchor the prosthesis and a pocket member fixed to the anchoring ring. The pocket member is positioned within the mitral valve, between the leaflets so that an open end of the pocket member is positioned within the left ventricle. When the mitral valve is open, blood flows past the pocket member, maintaining the pocket member in a collapsed state. When the mitral valve closes, the backpressure of the blood pushes into the pocket member, expanding the pocket member to an inflated shape. The mitral valve leaflets contact the expanded pocket member, allowing the prosthesis to block at least a portion of the openings between the leaflets, thereby minimizing regurgitated blood flow into the left atrium.