In all, 133 patients with an acute rupture of the anterior cruciate ligament (ACL) were reviewed (aged 40 to 59 years). Average follow-up was 29 +/- 10 months. Thirty-one patients underwent conservative therapy, 35 patients were treated by primary suture, while in 67 the primary suture was augmented with the semitendinosus tendon. The patients with primary repair and semitendinosus tendon augmentation showed significantly better results according to OAK and Lysholm scores, the Lachman test, pivot-shift testing, and KT-1000 arthrometer measurements than those treated conservatively or with primary suture. The physical activity level was significantly higher in the patients with augmented ACL repair than in the conservatively treated patients. There was no significant difference between the patients with augmented ACL repair and conservative treatment in the assessment of range of motion, while the patients with primary suture had a significantly greater loss of flexion than those in the other two treatment groups. Increasing age was not found to have a negative effect on either operative or conservative treatment. The results indicate that patients between the ages of 40 and 59 years can be treated successfully by ACL surgery.