This paper discusses a complication that is associated with mitral valve replacement utilizing the porcine heterograft and that, to the best of our knowledge, has not been reported previously. Severe mitral insufficiency developed early in 2 patients following mitral valve replacement with a porcine bioprosthesis. Both patients required reoperation. In each patient, it was discovered that one of the cusps of the mitral prosthesis was in the fixed-open position with no evidence of perivalvular leak. The assumption was that failure of the leaflet to close properly had been present from operation. Careful inspection of the valve and assurance that all leaflets close properly should be made at the time of initial valve replacement.