We sought to determine if knee stability after autogenous bone-patellar tendon-bone anterior cruciate ligament reconstruction was adversely affected by obtaining immediate full hyperextension. We selected patients based on degree of knee hyperextension. Group 1 (46 men and 51 women), with an average of 10 degrees (range, 8 degrees to 15 degrees) hyperextension, was compared with the randomly selected control Group 2 (70 men and 27 women), which had an average of 2 degrees (range, 0 degrees to 5 degrees) hyperextension. The operative knee in both groups, which underwent similar reconstruction of the injured knee, achieved full passive extension equal to the non-involved knee during the immediate postoperative course. The average KT-1000 arthrometer manual maximum side-to-side differences were 2.4 mm for Group 1 and 2.1 mm for Group 2 (P = 0.13). Seventy-nine patients in Group 1 had KT-1000 arthrometer differences of < or = 3 mm as compared with 85 patients in Group 2. Fourteen patients in Group 1 had KT-1000 arthrometer differences of 4 or 5 mm as compared with eight patients in Group 2. Four patients in each group had KT-1000 arthrometer differences > 5 mm. Evidence suggests that restoring and maintaining immediate full knee hyperextension after this type of reconstruction does not adversely affect the ultimate stability of the knee.