Subjects with knees osteoarthritis (KOA) show gait asymmetries evidenced by lower knee flexion and shorter contact times for the affected leg. Interestingly, running on a treadmill compared to running overground is also associated with lower knee flexion and shorter contact times. Thus, it is of particular interest how gait patterns are influenced by the type of ground in subjects with KOA. The aim of the current study was therefore to measure the overground asymmetry of kinematic parameters in KOA subjects while running and to investigate wether this asymmetry is altered on a treadmill. Nine patients diagnosed with KOA underwent overground and treadmill running with 3D-motion analysis. The symmetry analysis was performed using Symmetry Angles for five selected gait parameters: contact and step time, heel-toe delay, maximal knee flexion during stance and vertical speed variance. For all parameters, the values were significantly lower for the affected compared to the non-affected leg (p≤0.023). Post-hoc analyses revealed significant differences between legs only overground and not on the treadmill. The asymmetry was lower on the treadmill, as indicated by significant Symmetry Angle reductions for contact time (p=0.033), knee flexion (p=0.001) and vertical speed variance (p=0.002). The symmetry increase on the treadmill was mainly due to changes of the non-affected leg towards the affected leg values leading to smaller steps and less impact load in general. The present results suggest therfore taht a) an assessement of symmetry may differ depending on the ground type (treadmill versus overground) and b) treadmill running may be more suitable for patients with KOA gait asymmetries.