Abstract: A device is associated with the conventional prosthetic ring, which is permanently attached to the natural ring of the mitral valve and consists of two pieces attached to each other in a sliding arrangement, which form a positioning mechanism. One of the pieces has an essentially T-shaped form for its temporary attachment to the conventional prosthetic ring during the surgical operation, and it has a handle which sticks out from the piece on the side facing out from the prosthetic ring; and the other piece is in the form of an arc with a curvature corresponding to the opening of the mitral valve, and possesses an orifice at the mid-point of the arc, into which the trunk of the T-shaped piece is inserted in order to slide the arc-shaped piece in relation to the T-shaped piece and fix it temporarily over one of the edges of the mitral valve in order to perform the repair of the same.