In vitro testing of metal-metal hip implants has shown that metal-metal wear can be up to two orders of magnitude less than the wear of conventional metal- polyethylene articulations. This low wear may be related, in part, to fluid film lubrication at the bearing surfaces. A transient, elastohydrodynamic lubrication model incorporating both squeeze and entraining actions in the hip was developed to predict fluid film thickness for metal-metal hip implants during simulator testing. For a typical case, the model showed that cyclic steady state, with film thickness variations from 38 to 70 nm, was reached in about 3 cycles. Decreased diametral clearance, increased lubricant viscosity, and increased cycle frequency resulted in increased film thicknesses. However, film thickness did not change markedly for different load magnitudes. In all cases, steady state film thickness values calculated using the average load during the cycle corresponded to minimum cyclic steady state values, indicating that this may be a simplified method to predict minimum film thickness. Lambda ratio (ratio of fluid film thickness to combined surface roughness of articulating components) decreased as surface roughness increased, indicating a transition from full fluid film lubrication to boundary lubrication. Steady state lambda ratios were related to the total wear of simulator-tested implants with wear decreasing significantly with increasing lambda ratio. With proper selection of low clearance and low surface roughness values, lambda ratios that predict full fluid film lubrication and possibly further improved wear performance can be obtained, making lambda ratio a potentially useful tool in the design of metal-metal hip implants.