The importance of the anterior cruciate ligament (ACL) in relation to valgus-varus and axial rotation stability in the knee joint was investigated. Mobility patterns were drawn from ten osteoligamentous preparations after successive transection of the two parts of the ACL and the medial (MCL) and lateral collateral ligaments (LCL). The knee joint remained grossly stable after partial injury of the ACL, while sectioning of the entire ACL caused an increase in internal rotation in the extended-semiflexed position. Combined lesions to the ACL and the MCL caused considerable valgus instability increasing with flexion, the joints remaining stable in extension. Moreover, marked anteromedial instability occurred, while only slight posteromedial instability was found. Combined lesions to the ACL and the LCL caused varus instability, worst in the semiflexed position, and a consistent pivot shift in applying a valgus torque in flexion was noted. Moreover, moderate posterolateral instability was found, at its maximum in the semiflexed position. External rotatory stability is secured primarily by the MCL, secondarily by the posterior medial capsule, and finally by the ACL. The existence of lateral pivot shift is proof of damage to the ACL.