A review of the literature was conducted in an attempt to justify the exclusive use of either mechanical or tissue prostheses in the mitral position. In addition, the University of Toronto experience with mitral valve replacement was reviewed, including a five year follow-up from the Toronto General Hospital. Both studies concluded that there was no significant advantage for the use of either type of valve based upon freedom from thromboembolism, freedom from anticoagulant-related hemorrhage and freedom from all valve-related mortality/morbidity. Tissue valves were shown to be significantly poorer substitutes in terms of freedom from primary valve failure (P less than 0.05) and freedom from reoperation due to valve-related complications (P less than 0.07). The clinical results from the Toronto General Hospital correlated with those reported in the literature and suggested the preferential use of mechanical valves during mitral valve replacement based primarily on their durability and a consequent lesser need for reoperation.