We report a closed quadriceps tendon rupture in an 11-year-old girl. The lesion occurred when the girl jumped off a horizontal bar during a school gymnastic exercise; she sustained no direct trauma to the knee joint. At first, only hemarthrosis was diagnosed and the knee was immobilized in a cast. The patient continued to complain of knee pain and weakness of the knee extensors. The tendon rupture was not detected for 3 years. The quadriceps tendon was reinserted into the patella with restitution of knee function and only mild intermittent discomfort at follow-up 1 year later.