Background and Purpose. This cross-sectional study evaluated the relative contributions of psychosocial and mechanical variables to physical performance measures in people with knee osteoarthritis (OA). Subjects. Fifty-four subjects (age, in years: X̄=68.3, SD=8.7, range=50–87) with radiographically confirmed knee OA were included in this study. Methods. Physical performance measures included the Six-Minute Walk Test (SMW), the Timed “Up & Go” Test (TUG), and a stair-climbing task (STR). Responses to psychosocial questionnaires that reflect depression, anxiety, and self-efficacy (a person's confidence in his or her ability to complete a task) were collected. Mechanical variables measured included body mass index and knee strength (force-generatingcapacity of muscle). Stepwise linear regressions were performed with the SMW, TUG, and STR as separate dependent variables. Results. Functional self-efficacy explained the greatest amount of variance in all performance measures, contributing 45% or more. Knee strength and body weight also explained some variance in performance measures. Anxiety and depression did not explain any variance in performance. Discussion and Conclusion Physical therapists evaluating the significance of the SMW, TUG, and STR scores in subjects with knee OA should note that a large part of each score reflects self-efficacy, or confidence, for physical tasks, with some contributions from knee strength and body weight.