The relationship between the elongation values of an autogenous bone-patellar tendon-bone graft immediately after fixation and the anterior-posterior laxity of the knee 5 years later was studied in vivo. Immediately after fixation, the change in the graft midsubstance length during passive knee flexion-extension was measured using a Hall-effect transducer, and anterior-posterior knee laxity was measured with the KT-1000 arthrometer. Subjects were divided into group 1 (N = 6), with graft elongation values bounded by the 95% confidence intervals of the normal anterior cruciate ligament elongation values, and group 2 (N = 7), subjects with values outside these intervals. Immediately after reconstruction, the side-to-side difference in anterior-posterior laxity between the reconstructed and uninjured knees was not different between group 1 (-2.6 +/- 0.7 mm, mean +/- SEM) and group 2 (-1.7 +/- 1.0 mm) (P = 0.49). At 5-year follow-up, the difference was 1.2 +/- 0.7 mm for group 1, while for group 2 it was significantly greater at 4.7 +/- 0.6 mm (P = 0.004). At surgery, graft elongation values produced by flexion of the knee that are outside the limits of the anterior cruciate ligament result in significant increases in anterior knee laxity at long-term follow-up, while grafts with elongation values similar to the normal anterior cruciate ligament do not. Not only is restoration of anterior-posterior laxity values to within normal limits important, but the biomechanical behavior of the graft produced by flexion-extension of the knee should be appreciated.