Wear particle-induced osteolysis is widely regarded as the leading long-term complication associated with total hip arthroplasty (THA) . The need to reduce or eliminate UHMWPE debris has led to renewed interest in alternatives to the current metal-UHMWPE bearing couple, including a reevaluation of metal-on-metal designs. Previous analyses of both simulator-tested and retrieved metal-on-metal THA articulating components have shown that wear rates tend to decrease with a longer time in situ [2,3]. This phenomenon may result from an initial period of accelerated “run-in” wear, in which there is a partial removal of surface asperities and a correction of implant asphericity. This may lead to enhanced fluid film lubrication at the articulation interface and ultimately reduce long-term wear rates of the bearing components. The objectives of this study were to quantify the wear rates of a series of explanted Sivash metal-on-metal bearing components and to characterize changes in surface geometry configurations in order to better understand the running-in phenomenon associated with metal-on-metal bearing surfaces.