Seventeen fresh-frozen knee specimens from cadavera were instrumented with a load-cell attached to a mechanically isolated cylinder of subchondral bone containing the tibial insertion of the anterior cruciate ligament. The forces in the intact anterior cruciate ligament were recorded as the knee was passively extended from 90 degrees of flexion to 5 degrees of hyperextension without and with several constant tibial loads: 100 newtons of anterior tibial force, ten newton-meters of internal and external tibial torque, and ten newton-meters of varus and valgus moment. The anterior cruciate ligament was resected, and a bone-patellar ligament-bone graft was inserted. The knee was flexed to 30 degrees, and the graft was pre-tensioned to restore normal anterior-posterior laxity. The knee-loading experiments were repeated at this level of pre-tension (laxity-matched pre-tension) and at a level that was forty-five newtons greater than the laxity-matched pre-tension (over-tension). During passive extension of the knee, the forces in the graft were always greater than the corresponding forces in the intact anterior cruciate ligament. Over-tensioning of the graft increased the forces in the graft at all angles of flexion. At full extension, the mean force in the anterior cruciate ligament was fifty-six newtons; the mean force in the graft at laxity-matched pre-tension was 168 newtons, and it was 286 newtons in the over-tensioned graft. Greater pre-tensioning may be required when the knee demonstrates apparent tightening of the graft in flexion. The mean forces in the graft generated during all constant loading tests were greater than those for the intact anterior cruciate ligament over the range of flexion. When the graft was over-tensioned, the forces generated by the anterior tibial force and by varus and valgus moment increased but those generated by internal and external tibial torque did not. There was no significant change in the mean tibial rotation as a function of the angle of flexion of the knee after insertion of the graft; normal tibial rotation of the knee during passive extension (the so-called screw home mechanism) was eliminated.CLINICAL RELEVANCE: When a patellar ligament allograft was pre-tensioned to restore normal anterior-posterior laxity, the forces in the graft were markedly greater than those in the intact anterior cruciate ligament. Thus, the penalty of increased forces in the graft must be accepted if anterior-posterior laxity is to be restored. Of particular concern are the large forces in the graft generated by loading states, such as external tibial torque and varus moment, which normally generate minimum force in the intact anterior cruciate ligament. In terms of force magnitude, internal torque applied to an extended knee is likely to be the most dangerous loading state for a patient who has a patellar ligament graft. There is a current trend toward early postoperative mobilization and intensive rehabilitation after substitution of the anterior cruciate ligament with a graft. Although this approach results in an excellent range of motion, the surgeon should be aware that a return to full activity could produce forces in the graft that are many times greater than those in the intact anterior cruciate ligament. For this reason, early return to full activity may not be indicated until full biological maturation of the graft.