Ten Lillehei-Kaster heart valve prostheses, in situ for up to 10 years and recovered at surgery or necropsy, were examined by light and scanning electron microscopy. All showed metal wear on the luminal aspect of their struts. The volume of wear related to the duration a prosthesis had been in situ. The worn metal showed distinct, transverse surface corrugations, which became more obvious with time. Aortic prostheses wore more and faster than mitral ones. One strut usually showed more wear than the other, a change likely due to specific manufacturing methods. It is believed that the pattern of wear is caused by a velocity-controlled stick-slip abrasive wear process, resulting from an interaction between the edge of the moving pyrolytic carbon disc, the struts' titanium surface, and the protein coat covering that surface. None of the patients had prosthesis dysfunction attributable to metal wear. Disc escape seems unlikely considering the degree of wear observed after 10 years. Furthermore, the surface corrugations did not appear to cause disc sticking or other problems. However, clinicians might consider monitoring patients who have borne these prostheses for greater than 10 years.