This study examined the biomechanics of anterior cruciate ligament-patellar tendon autografts for as long as 3 years after surgery. Twenty-seven adult female goats were tested; four served as controls and the others received an autograft to the right knee with each left knee serving as an additional control. The animals with grafts were tested at 0 week (n = 4), 6 weeks (n = 4), 12 weeks (n = 4), 24 weeks (n = 3), 1 year (n = 5), and 3 years (n = 3) after surgery. The anteroposterior laxity of the knee joint, load-relaxation, and structural and material properties of the graft were tested. The anteroposterior laxity was significantly greater than that of the controls for all groups except at 3 years. Load-relaxation was greater than that of the control anterior cruciate ligaments, but in the 1 and 3-year grafts load-relaxation was less than that of the patellar tendons with 5 minutes of sustained loading. Between 12 and 52 weeks, the stiffness and modulus of the grafts increased 3-fold, but the improvement was slow afterward. At 3 years, the strength and stiffness of the grafts were 44 and 49% those of the control ligaments, respectively; the modulus was 37 and 46% that of the control anterior cruciate ligaments and patellar tendons, respectively. The persistent inferior mechanical performance at 3 years suggests that anterior cruciate ligament grafts may never attain normal strength.