In military training, the knee joint is more susceptible to trauma than any other joint in the body. Chronic knee instability is a difficult problem for both the patient and the orthopaedic surgeon who must correct the disability. To achieve the best possible result, the surgeon must first make the correct diagnosis and be aware of the type or types of instability that exist. There may be associated instability in a rotatory plane as described by Slocum and Larson, and the surgeon must examine carefully for this. Various surgical techniques have been used in the past to repair the knee static stabilizers such as ligaments and capsule. Since the introduction of the dynamic concept of rotatory instability and the need for muscular reinforcement of static repair, most orthopaedists have attempted to back up their reapirs with various types of dynamic stabilization such as the pes anserinus transfer and advancement of semimembranosus. 39 cases of medial instability of the knee joint were analysed clinically in the department of orthopaedic surgery of Busan Armed Froce General Hospital from Feb. 1977 to Feb. 1980 and following results were obtained. 1. All of the cases were military personnels, most of them were privates and P.F.C. showed most frequent incidence in 19 cases (49%). 2. Sports injury was a main causes of the knee instability. 3. The highest incidence in severity of the lesion was grade III in 21 cases (54%). 4. Among 39 cases of ligamentous injuries of medial side, rupture of collateral ligament alone were 30 cases, and mixed injuries with cruciate were 9 cases. 5. The pes transfer was performed in all of the cases with acute medial collateral rupture, and even in the cases with instability after primary repair, good results were obtained by carrying out dynamic muscle or tendon transfer. 6. In some cases, the patient did not feel much discomfort for the remaining static instability in valgus stress test after dynamic reconstruction. This was thought to be overcome by dynamic force during use of the leg. 7. For the purpose to prevent further stretching of repaired ligaments, derotation brace were recommended in 5 cases of dynamic reconstructive surgery.