Polycrystalline diamond (PCD) bearings are designed for use in harsh environments, including process-fluid-lubricated applications such as those in oil and gas drilling turbines. This paper discusses the wear-in behaviour of polycrystalline diamond thrust bearings. Laboratory test results are presented that show the roughness, profile, and coefficient of friction of the polycrystalline diamond wear pads as the wear-in of the polycrystalline diamond bearings progresses. Laboratory test results show a significant decrease in the surface roughness and a change in the profile of the wear pads. Measured values of the coefficients of friction reveal that the lubrication regime of PCD thrust bearings can move from boundary to mixed-mode and may even become hydrodynamic after wearing-in for a period of time under the right conditions. This mechanism can provide extended life for PCD thrust bearings in harsh environments.