Objective: To determine the validity of varus-valgus motion as a measure of knee joint stability by establishing the relationship of varus-valgus motion with muscle strength, joint proprioception, joint laxity and skeletal alignment in patients with knee osteoarthritis (OA). Methods: Sixty-three patients with OA of the knee were tested. Varus-valgus motion was determined with a video-based optoelectronic gait analysis system. Muscle strength was measured using a computer-driven isokinetic dynamometer. Proprioceptive acuity was assessed by establishing the joint motion detection threshold in the anterior-posterior direction. Laxity was assessed using a device which measures the passive angular deviation of the knee in the frontal plane. Alignment was assessed using a goniometer. Regression analyses were performed to assess the relationship between varus-valgus motion, muscle strength, joint proprioception, joint laxity and skeletal alignment. Result: Varus-valgus motion was not related to muscle strength, joint proprioception, joint laxity and skeletal alignment. Conclusions: Knee joint stability cannot be measured as varus-valgus motion. Rather, a number of independent factors seem to contribute to the process of stabilization of the knee joint. © 2007 Osteoarthritis Research Society International.