Background: The effect of injury to the posterolateral structures of the knee on the success of an anterior cruciate ligament reconstruction is not well known. Hypothesis: Increasing graft tension increases the amount of external rotation of the tibia if the posterolateral structures are deficient. Study Design: Laboratory study. Methods: Eight cadaveric knees underwent techniques similar to a clinical reconstruction except that the distal fixation on the tibia was an external tensioning device used to apply a traction force on the graft. The knee was secured in a joint-testing machine and an instrumented spatial linkage was used to measure the motion of the tibia with respect to the femur. Measurements were taken with forces increasing from 0 to 100 N. The fibular collateral ligament, popliteofibular ligament, and the popliteus tendon were individually cut sequentially, and differences in the relative position of the tibia with respect to the femur were compared with the intact baseline. Results: External rotation increased significantly when all of the posterolateral structures were cut and 60, 80, or 100 N of distal traction was applied. Conclusions: Deficiency of posterolateral structures of the knee significantly affected the relative external rotation of the tibia. Clinical Relevance: Injured posterolateral structures should be repaired before fixation of anterior cruciate ligament grafts.