The On-X prosthetic valve was developed in 1996. Compared to other bileaflet prosthetic valves, it provides better hemodynamics and fewer complications. We report a case of aortic stenosis, in which severe mitral regurgitation occurred after aortic valve replacement with On-X valve. Transesophageal echocardiography revealed that the mitral valve had been distorted by the long body of the On-X valve. We replaced it with an Edward Mira mechanical valve and the mitral regurgitation returned to its preoperative level.