We report four patients with a mean age of 17 years (14 to 22) with external rotation injuries of the knee in slight flexion. Radiographs showed a small fragment in the area of the lateral femoral condyle. At operation, the fragment, consisting of the femoral insertion of the popliteus, was anatomically reduced and fixed. At a mean follow-up of 35 months all the knees had an excellent function score. An isolated lesion of the popliteus often presents as a tendon avulsion whereas major damage to the posterolateral corner of the knee involves combined ligamentous injuries. In patients with an acute haemarthrosis and lateral pain in a stable knee, the diagnosis of isolated avulsion of the popliteus tendon should be suspected. Arthroscopy with special attention to the lateral gutter is indicated. We advise anatomical reduction and fixation of the fragment to prevent possible long-term effects on other posterolateral structures.