Metal-on-metal bearings have wear rates that are 20 to 100 times lower than metal-on-conventional polyethylene. The amount of wear generally is the same order of magnitude for the head and the cup. There is an initial run-in period of higher wear followed by lower, steady-state wear. Wear rate is a function of the interplay of material(s), macrogeometry, microgeometry, and the resultant type and amount of lubrication. The wear resistance and clinical performance of a metal-on-metal bearing are more sensitive to macrogeometry and lubrication than a metal-on-polyethylene bearing. Metal wear particles are nanometers in linear dimension. They are much smaller and more numerous than the submicron polyethylene wear particles, but the volume of periprosthetic inflammatory tissue is less. Osteolysis seems to be relatively rare. Little is known about the systemic distribution of metal particles and ions. The significance of systemic distribution also is not known. The levels of serum and urine Co and Cr ions are elevated in patients with metal-on-metal bearings, but the long-term, steady-state levels are not much higher than those from corrosion of modular femoral components. Because of the elevated levels of Co and Cr ions, there is a greater risk of delayed type hypersensitivity. There also is concern about the potential for malignant degeneration secondary to prolonged exposure to these elements. The available data are insufficient to address this concern. Rigorous long-term studies are needed. It will take decades of close clinical observation to determine if the benefits of metal-on-metal bearings outweigh the associated risks.