AbstractIt is generally agreed that contact pressure and sliding speed are the predominant factors for the prediction of wear of ultrahigh‐molecular‐weight polyethylene (UHMWPE) in joint prostheses. A new parameter for predicting the wear of UHMWPE has been introduced with a wear test in vitro. The parameter is the time of exposure to a lubricating liquid on a bearing surface. A pin‐on‐disc machine was designed such that the exposure time of a CoCrMo alloy disc to a lubricating liquid could be varied. The specific wear of UHMWPE was increased by a decrease in the exposure time, even if the contact pressure and the sliding speed were held constant. The parameter is able to account for the contact pressure set in the experiment (2.0–20.0 mPa), and clarifies the conditions under which the specific wear of UHMWPE is fou