Between January 1980 and February 1987 92 patients (group I) with chronic anterior instability of the knee joint underwent anterior cruciate ligament reconstruction using midthird patellar tendon autografts (BTB). 39 patients (group II) with the same injury were treated conservatively. The patients were followed up postoperatively within a mean period of 8.5 years and stability, grade of postoperative arthrosis, subjective view of patients and ability to sport were evaluated. In group I the clinical examination showed in 77% the same stability on both sides, whereas in group II no cases of bilateral stability of same grade were found. Pivot shifting was repaired in 91% of group I and only in 17% of group II patients. The apparative examination of the stability (KT 2000-arthrometer testing) measuring the anterior drawer phenomenon on both knees showed stable joint conditions (anterior tibia displacement < or = 3 mm) in 86% of operatively treated and only in 8% of conservatively treated patients. Postoperative x-rays showed no arthrotic changes in 55% of group I patients. All patients of group II showed signs of progredient arthrosis in their postoperative x-rays. Subjective clinical results were good to excellent in all group I patients. Only 50% in group II had a good to excellent result. 67% of group I and only 25% of group II patients practiced their former sports activities again. In our retrospective review we could observe significant better clinical results of surgically treated old isolated anterior cruciate ligament (ACL) tears with patellar tendon autografts (BTB) compared to those with conservative treatment.