Arthroscopy was performed during the acute phase of injury in 84 knees (79 patients). A satisfactory view of the joint was obtained in all cases, and no complications occurred. About two-thirds of the patients had injuries associated with violent rotation-abduction. In about one-third of the patients operation could be avoided. In cases with haemarthrosis, serious ligament injury was present in nearly 50 per cent. Complete arthroscopy was associated with few diagnostic errors. Clinical examination often led to uncertain or incorrect diagnosis even when performed under anaesthesia by experienced surgeons. In contrast, arthroscopy led to rapid diagnosis and treatment, thus shortening the period of disability. We recommend arthroscopy in acute knee injuries, but the examination must be performed by an experienced arthroscopist.