Within two months of a knee injury involving valgus and external-rotation stress or rapid deceleration, twenty-one patients had arthroscopy. Despite blood in the joint (nineteen had hemarthrosis), useful information for diagnosis and treatment was obtained in eighteen patients. A characteristic deformity of the meniscus indicative of rupture of its peripheral attachment was identified and the extent of injury to the capsular ligaments could be assessed. It was also possible to visualize isolated tears of the anterior cruciate ligament and to identify those that might be repaired surgically.