Objective To study the diagnosis and treatment of the acute medial collateral ligament ruptures of the knee.Methods From August 1998 to August 2003, 87 cases of acute medial collateral ligament ruptures were examined with physical method and MR imaging. Out of them, 35 cases of Ⅰdegree and Ⅱ degree ruptures were treated with non-surgery and 52 cases of Ⅲ degree ruptures were treated surgically. The torn medial collateral ligaments were mended, 21 of which were strengthened with the anterior partial gracilis muscle tendon after the arthroscopy. Results In 35 cases of Ⅰ and Ⅱ degree ruptures, 32 were followed up 13 months on average. According to Lysholm scoring system, the clinical results were classified as excellent or good in 93.7% of the cases. In 52 cases of Ⅲ degree ruptures, 50 were followed up 16 months on average. The excellent or good result was 90%.Conclusion For Ⅰ and Ⅱ degree ruptures, MR imagimg is an important way to definitely- diagnose medial collateral ligament ruptures. Abduction stress test of knee extension shows that the medial direct instability is a main way to definitely diagnose Ⅲ degree ruptures. The results of conservative treatment of Ⅰ degree and Ⅱ degree ruptures are excellent. Surgical therapy are fitfor the cases of Ⅲ degree ruptures.