Abnormal motion of the foot has been suggested to be a potential contributor to the medial collapse of the knee seen in individuals with patellofemoral pain syndrome during a one-leg squat. Rearfoot frontal plane range of motion and peak angle were investigated during a one-leg squat in patients with patellofemoral pain syndrome compared with controls. A significant increase in the peak eversion angle was found for the patellofemoral pain syndrome group compared with controls (P = .02). Increased rearfoot eversion may contribute to the knee medial collapse observed in these patients because it may lead to tibial abduction relative to the femur. Nevertheless, further research is required to investigate the causal relationship because it remains unclear whether rearfoot eversion instigates the medial collapse of the knee.