A 26-year-old active woman with knee pain was seen by her primary care physician and diagnosed with patellofemoral pain (PFP). Radiographs of the knee were interpreted as noncontributory. Her knee symptoms continued, and 8 weeks later she was referred to a physiatrist, who confirmed the PFP diagnosis and referred her to physical therapy. Repeat radiographs revealed a destructive mass involving the diaphysis of the femur and surrounding soft tissue. Subsequent magnetic resonance imaging further characterized the mass as an aggressive bone lesion, and biopsy confirmed that it was Ewing sarcoma. J Orthop Sports Phys Ther 2018;48(7):594. doi:10.2519/jospt.2018.7835