Since Palmer's introduction of the torn medial collateral ligament, many clinical and anatomical studies were performed. But any reconstructive procedure of the medial collateral ligament of the knee known to us, has not solved the problem completely yet. The purpose of present study is to describe a technique of reconstruction, socalled Tatsuzawa's tenodesis surgery of transposed semitendinosus tendon, and is to report the clinical results of 45 cases treated by his procedure between April 1976 and June 1981. In addition, significance of the procerdure was compared with the result of primary closure group of the torn ligament. Tatsuzawa's procedure consists of reinforcement surgery of the repaired or unrepaired torn medial collateral ligament with forwardly transposed semitendinosus. The semitendinosus tendon was brought forward to lie adjacent to deep layer of the medial collateral ligament. The transposed part of the tendon was sutured throughout its length to the medial collateral ligament and to the medial aspect of the tibia and femoral condyle. Because the direction of transposed tendon as similar to the direction of fibers of medial collateral ligament, the procedure was very simple and anatomical one, and had some functional advantages. In this series some old cases are treated only with tendon reinforcement operation without repairing the laxed torn ligament. The results of the surgery were evaluated by the Asais modification of the Feltons evaluation criteria. Results obtained were as follows: 1. In semitendinosus tenodesis with or without primary repair of the torn ligament, satisfactory results were obtained (excellent and good) in 33 cases (73.3%). But in 20 cases of the primary repair group, the satisfactory results were obtained in 11 cases (55%). More satisfactory results were obtained with the semitendinosus tendon reinforcement operation of the torn ligament than only with primary repair. 2. In 29 cases with fresh injury of the medial collateral ligament in which the primary repair together with the tendon reinforcement procedure was performed, satisfactory (excellent and good) results were obtained in 72.4% (21 cases), and in 16 untreated old cases in which only the tendon reinforcement procedure were done, satisfactory results were obtained in 75%. 3. In 5 cases having medial collateral and anterior cruciate ligaments tear, excellent results were obtained in 3. This explains that semitendinosus tendon reinforcement procedure is the key one of the reconstructive operative procedures. It is thought that the procedure is also effective for the anterior cruciate tear to provide the anterior instability.