This project was supported by a grant from the Physiotherapy Foundation of Canada. The purpose of this study was to determine the reliability and validity of the following outcome measures in a group of 18 patients with patellofemoral pain syndrome: the visual analog pain scale (VAS), a functional index questionnaire (FIQ), selected temporal components of gait on level walking and ascending stairs, knee joint angle on downhill walking, and electromyographic activity of the quadriceps during stair climbing. Subjects were tested at initial assessment (time 0), after 24 hours (time 1), and after clinically significant improvement, following a course of treatment (time 2). Using the intraclass correlation coefficient (r1), the VAS (r1 = 0.603) and FIQ (r1 = 0.483) exhibited poor day-to-day reliability (time 0 versus time 1). However an ANOVA between time 0 and time 2 showed them to be valid measures for detection of clinical change (p < 0.01). No differences in the gait variables were observed from time 0 to time 1 or time 2, suggesting that gait analysis may not be sensitive enough to detect changes in pain and function in this patient population. J Orthop Sports Phys Ther 1989;10(8):302-308.