Posterolateral instability of the knee is difficult to diagnose and treat. It has been attributed to failure of ligament reconstruction and has been the cause of numerous knee operations. We present a small group of patients who complained of giving way of the knee and who had an increased range of external rotation of the tibia at 90 degrees knee flexion. The patients all had similar symptoms. We describe the standing apprehension test, which was positive in every case. Anterior subluxation of the lateral femoral condyle was detected manually in four patients and confirmed by radiography and MRI in one. The medial knee pain which is often associated with episodes of instability is probably due to stretching of the posteromedial soft tissues and perhaps the saphenous nerve.