A 9-year-old girl presented with knee pain and inability to extend her leg after landing on her flexed right knee while jumping on a trampoline. Physical examination revealed superior displacement of the patella (Panel A), and a radiograph showed a fracture of the patella at the inferior pole (Panel B; arrows indicate the two points of fracture). Fractures of the patellar sleeve occur when a “sleeve” of periosteum is pulled off the patella. These fractures occur almost exclusively in children, before the patella fully ossifies, and usually occur in response to the pressure of a forced load on a flexed knee; the majority involve the inferior patellar pole. The fracture may be difficult to see on a radiograph, especially when the fragment of avulsed bone is small or absent. In this patient, the diagnosis was confirmed on the day of injury during open reduction and internal fixation. The patient was able to return to full activity within 2 months after the injury occurred.