Wear of titanium 6-4 alloy against UHMW polyethylene was compared to that with cobalt chromium or stainless steel, in several types of laboratory wear machines and on retrieved failed hip prostheses. In the absence of third-body abrasive contaminants, titanium alloy tended to produce polyethylene wear equal to and sometimes lower than the other alloys. Severe, self-perpetuating abrasive-corrosive wear of the titanium alloy could be initiated by the entrapment of third-body particles between the metal and polymer, sometimes with particles of acrylic surgical cement and always with metallic particles, as from porous coated prostheses. The micro-scratching (dulling) commonly observed on the bearing surfaces of retrieved titanium alloy components appeared to be primarily due entrapment of third-body acrylic and/or metallic particles that were initially generated by fretting of loosened prostheses, for example, at the stem-cement interface. Nitrogen hardening of the titanium surfaces, e.g., by diffusion or ion-implanting, provided additional protection against cement particles, but did not prevent severe abrasion by entrapped metallic particles.