STUDY DESIGN: Pretest-posttest matched control group design. OBJECTIVES: To measure passive knee joint stiffness and pain in participants with and without patellofemoral pain syndrome (PFPS) and to determine the relationship between mechanical knee joint stiffness, self-reported stiffness, and pain. BACKGROUND: Patients with PFPS complain of knee joint stiffness and pain, but no research has quantified both of these characteristics in this population. METHODS AND MEASURES: Twenty-eight individuals (14 with PFPS [mean age +/- SD, 25.5 +/- 4.8 years] and 14 healthy controls [mean age +/- SD, 22.8 +/- 5.4 years]) volunteered for this study. Mechanical passive knee joint stiffness was calculated using the damped natural frequency of oscillation of the lower leg while sitting. Mechanical stiffness was compared to self-reports of knee stiffness and pain. All measurements were recorded presitting and after 20 minutes of sitting. RESULTS: Sitting for 20 minutes did not induce significant changes in mechanical knee joint stiffness. However, participants with PFPS reported significantly greater (P<.01) knee stiffness after sitting for 20 minutes. A significant correlation (r = 0.70, P<.01) was found between self-reported stiffness and pain in participants with PFPS; however, no significant relationship was observed between mechanical and self-reported knee joint stiffness. CONCLUSIONS: Despite frequent complaints of joint stiffness, the knees of individuals with PFPS do not appear physiologically stiffer than those of control subjects. Individuals with PFPS perceive increased knee stiffness after sitting, but may misinterpret the sensation of pain as joint stiffness.