Wear induced osteolysis is cited as the primary cause of aseptic loosening in knee replacements. It has been postulated that mobile bearing (MB) knee prostheses reduce wear as they allow lower contact stress through high congruency between components while maintaining a wide range of joint motion. In vitro wear simulations are preclinical tests for predicting the performance of new designs of partial and total knee replacements. This study investigates the wear of a leading design of MB uni-compartmental knee replacement (UKR) by quantifying the in vitro wear rate and linear penetration of the polyethylene meniscal bearing. Three medial and three lateral MB UKRs (Uniglide, Corin, Ltd., U.K.) were tested in a three station wear simulator using force control as defined in ISO 14243-1. Volumetric wear was determined gravimetrically every half million cycles (MC) up to 10 MC. Maximum linear penetration wear was measured after 10 MC. Volumetric wear rates of 1.65 ± 0.28 mm³/MC (mean ± SD) and 1.66 ± 0.31 ³/MC were recorded for the medial and lateral bearings, respectively, and the wear of all bearings was linear. Previous simulator studies have reported mean wear rates of 3.8 to 10.4 ³/MC for MB UKRs. Maximum linear penetration wear rates of 0.013 ± 0.001 mm/MC and 0.012 ± 0.001 mm/MC were recorded for the medial and lateral bearings, respectively, after 10 MC. This was in agreement with that reported for well functioning MB UKRs through in vivo measurement (0.01 mm/year) and compared favourably to that reported for fixed bearing designs (0.15 mm/year). The results of this study show that in vitro measurements correlate well to in vivo measure-ments of wear. This indicates that the current in vitro methodology for the simulation of wear in MB UKRs is a valid tool for preclinical assessment of MB UKR prostheses.