Rotatory Instability of the permitting abnormal external totation of the tibia on the femur is the result af forced abduction of the flexed knee and external rotation of the tibia. The basic lesion is a tear in the medlal capsular ligament. The authors have reported clinical study in 40 patients with medial collateral ligament injuries who were treated by operative methods in the Orthopedic Department at the Taegu Presbyterian Hospital from January 1973 to Octdber 1979. of 40 patients, pes anserinus transplantation was performed on 15 patients and direct repair on 25 patients. The results are as followings: 1. All the patients were seen with a complete rupture of the medial collateral ligament and of these 45% had rupture of the femoral attachement. 2. Exellent to good result were more often obtained in recent injuries of less than 2 weeks rather than those in older Injuries. 3. Exellent to good result were more often obtained in the groups of Pes Anserinus tranplantation than the groups of direct repair. 4. In all cases of groups of pes anserinus transplantation, permanent sensory palsy af the patellar branches of the saphenous nerve was present due to the location of the surgical incision.