The primary purpose of the anterior cruciate ligament (ACL) is to restrict the anterior translation of the tibia on the femur. The ACL is frequently injured causing an active person to complain of instability, which may develop into arthritis. To regain stability and decrease the chance of arthritis, ACL reconstructions are performed. However, recurrent anterior instability occurs in 10-20% of patients following surgery. This instability manifests as an increase in anterior laxity, which is a symptom of a poorly functioning graft construct (bone-fixation-graft). One of the potential causes of the increase in anterior laxity is lengthening at the sites of fixation. Previous in vitro studies have demonstrated a millimeter-for-millimeter relation between lengthening at the sites of fixation and an increase in anterior laxity; however, in vivo the relation remains controversial because other factors such as biological incorporation might increase the stiffness and decrease the contribution to anterior laxity. The primary objectives of this dissertation were 1) to develop and validate methods to measure lengthening at the sites of fixation and the increase in anterior laxity and 2) to apply these methods in an in vivo study to determine whether the increase in anterior laxity following ACL reconstruction can be attributed to lengthening at the sites of fixation. To satisfy these objectives, Roentgen stereophotogrammetric analysis (RSA) was used to measure the lengthening at the sites of fixation and the increase in anterior laxity. For the first objective, tantalum markers were implanted into the bones, graft, and fixation devices during ACL reconstruction on 6 cadaveric legs. The knees were cyclically loaded in the anterior/posterior direction for 1500 cycles and both the lengthening at the sites of fixation and the increase in anterior laxity were measured. Because the lengthening at the sites of fixation and the increase in anterior laxity were determined successfully using RSA in vitro, similar methods were used in an in vivo longitudinal study. Nineteen subjects were treated with a soft-tissue allograft and slippageresistant fixation. Tantalum markers were injected into the bones, graft, and fixation devices. On the day of surgery and at 1, 2, 3, 4, 6, and 12 months, RSA was used to measure lengthening at the sites of fixation and the increase in anterior laxity. It was determined that although lengthening at the sites of fixation did occur, it was not related to the increase in anterior laxity and that it was offset by an increase in stiffness at the sites of fixation most likely due to biological incorporation of the graft into the tunnel wall.