The diagnosis of chronic injury to the anterior cruciate ligament is proved by accurate interpretation of its visualization at arthroscopy, arthrotomy, or necropsy. Chronic injury is suspected if the history indicates an original trauma with an episode of swelling or hemarthrosis followed by a temporary period of disability and a subsequent report of repeated limb collapse. Positive functional tests such as the crossover, leaning hop, and deceleration tests support the diagnosis, as does the radiographic presence of a Segond's fracture or a notch in the lateral femoral condyle. The diagnosis is confirmed by the combined presence of positive laxity and instability tests. The laxity tests employed are the drawer tests with the knee in 15 and 90 degrees of flexion and the flexion-rotation drawer test. In diagnosing chronic injury to the anterior cruciate ligament, I place most reliance on the presence of positive pivot shift tests that demonstrate instability. Occasionally, a direct radiograph of the subluxated knee joint or double contrast arthrography is employed to confirm the diagnosis.