A 10-YEAR-OLD Hispanic boy fell from a swing, had medial joint pain of the lower left extremity, and was unable to fully extend the knee. Plain radiography of the left lower extremity showed no evidence of fracture or dislocation. Arthroscopy revealed an old anterior cruciate ligament tear and exuberant synovium; biopsy findings showed fibrosis. Persistent inability to fully extend the knee and proximal tibial tenderness prompted repeated radiographic evaluation. Anterior, posterior, and lateral plain radiographs of the left tibia (Figure 1) revealed an ill-defined lytic lesion in the proximal tibial epiphysis. Soft tissue swelling was appreciated at the joint space on lateral view. On a T1-weighted magnetic resonance image, the lesion was crossing the growth plate and extended into the metaphysis. It was centrally located but extended laterally and medially to the epiphyseal and metaphyseal margins (Figure 2). The patient underwent left tibial biopsy (Figure 3).