This study is an analysis of the clinical examination of 118 consecutive patients with severe, acute injury of the knee joint, that is, with intra-articular effusion or with clinical suspicion of ligamentous instability. Both the clinical examination and the examination under anaesthesia for ligamentous instability involve a large number of falsely positive and falsely negative diagnoses. The greatest uncertainty is attached to the examination of the anterior cruciate ligament and menisci. The diagnostic specificity and the diagnostic sensitivity of the clinical examination of the anterior cruciate ligament are 0.75 and 0.62 respectively. The corresponding values for examinations under anaesthesia are 0.86 and 0.76 respectively. Two-thirds of the patients without intra-articular effusion, whose clinical examination gives rise to suspicion of ligamentous instability, however, have subclinical intra-articular haemorrhage and have as many cruciate and combined lesions as the whole series, viz. respectively 63 and 47 per cent. On the basis of this we conclude that arthroscopy should be carried out in all patients with a traumatic haemarthrosis or indication of ligamentous instability.