The Starr-Edwards (S-E) mitral valve, Model 6120, has been widely used in the last three decades. Fatty infiltration and subsequent damage of the silastic ball, a phenomenon termed ball variance, has been described only in the first generation aortic ball-valve prostheses, used until 1965. A case of partial ball fracture in a 6120-Model S-E mitral valve prosthesis, occurring 27 years after implantation and presenting with thromboembolism and progressive valve dysfunction, is described.