The large diameter metal-on-metal hip prostheses have sometimes shown erratic, unpredictable wear behavior in hip simulator tests. It was shown in the present study that high variation in wear is due to the fact that the conditions approach the tribological endurance limit of the metal-on-metal device in question. When the limit is exceeded wear is high and linear, and its variation is low. Below the limit under ‘normal’ test conditions, both the wear and its variation are low. Potential clinical wear problem scan be predicted by hip simulator studies provided that relevant adverse test conditions are included.