In between 1983 and 1988, 33 patients suffering from an acute antero-medial instability of the knee were operated on for the anterior cruciate ligament. The reconstruction of the ligament was performed by transcondylar fixation. 25 of these patients took part in a retrospective examination. The postoperative treatment had been carried out according to various concepts of rehabilitation. Our investigations were supposed to measure the restoration of the muscular efficiency of the knee-extensor and -flexor by isokinetic training arrangements. The results revealed that by the operation a sufficient stabilization of the knee had not been achieved. 18 of 25 patients once more showed an antero-medial instability of the knee. Twelve of these 25 patients had an extended anterior instability. Only in seven cases the muscular system was completely restored with average isokinetic muscle efficiency. The high rate of postoperative knee-instability proves the persistent sports-incapacity. We must draw the conclusion that the after-treatment of ligamentous injuries about the knee must focus on the muscular rehabilitation.