Patients suffering from a rupture of the anterior cruciate ligament (ACL) with instability of the knee joint are treated with an operative ACL-reconstruction: one group of patients with a reconstruction with an autograft tendon (hamstrings), the other group with a reconstruction with an allograft tendon (tibialis posterior). The same surgical technique, the same fixation technique and the same rehabilitation protocol will be used in both groups. Follow-up will be done during 2 years.