The surface of ground gear teeth is changed during the initial period of operation through a process termed running-in. During this process asperity peaks are plastically deformed and removed to better distribute the load across the surface, resulting in modification of the surface finish. In this work the influence of pressure, slide-roll ratio, and entrainment velocity on two-dimensional surface roughness parameters is evaluated through the running-in process using a full-factorial experimental programme. Hardened steel disks are used to simulate gear tooth contacts via the use of a twin-disk rig. Results showed that all three variables strongly influence the change in surface geometry, both individually and through both two- and three-factor interactions.