A medical system (300) is disclosed for treating a defective mitral valve (MV) having an annulus (A). The system (300) comprises in combination a removable and flexible elongate displacement unit (301) for temporary insertion into a coronary sinus (CS) adjacent the valve, wherein the displacement unit has a delivery state for delivery into said CS, and an activated state to which the displacement unit is temporarily and reversibly transferable from the delivery state. The displacement unit comprises a proximal reversibly expandable portion (302), a distal anchoring portion (303) being movable in relation to the proximal expandable portion in a longitudinal direction of the displacement unit to the activated state in which the shape of the annulus is modified to a modified shape (A')； and an annuloplasty device (102) for permanent fixation at the mitral valve annulus by annuloplasty of the valve when the modified shape is obtained. The annuloplasty device (102) comprises a fixation structure that is adapted to retain the modified shape.
