To study the effect of the pretension level in reconstructions of the anterior cruciate ligament (ACL) we examined 26 patients with isolated ACL insufficiency and intact Dacron ligament prosthesis at 4-year follow-up. The patients were divided into two groups of 13 each, based on pretension level applied at the reconstruction: in group I the ligament was pretensioned to 60 N and in group II to 40 N. The patients were evaluated by the Tegner and Lysholm scores, KT-1000 arthrometer measurements, and isokinetic muscle performance testing and were assessed by the Knee Ligament Standard Evaluation Form of the International Knee Documentation Committee. At follow-up there were no differences between the two groups in any of the evaluated parameters except for squatting ability. The two groups differed significantly (P < 0.01) with regard to the squatting score: 11 patients in group I had decreased squatting ability, and 6 of these were not able to squat beyond 90 degrees of flexion. In contrast, only 4 patients in group II had slightly impaired squatting ability. It is likely that this difference between the two groups is related to the magnitude of the pretension applied to the ligament prosthesis, and that the pretension exerts its influence through a change in the knee kinematics introduced at the time of the reconstruction.