The medial collateral ligament is the most commonly injured ligamentous structure of the knee. The management of the medial collateral ligament injuries were divided into surgical and conservative treatment. More recently, many investigators reported that isolated medial collateral ligament injuries did well under non-operative treatment. So, arthroscopic examination of the knee to rule out other intraarticular pathologic conditions such as cruciate ligament injuries, meniscal tear and osteochondral lesion is essential. From October 1993 to April 1995, we treated 45 cases of Grade III medial collateral ligament injuries and arthroscopic finding for cruciate ligament, meniscus, bony structure were analized in these cases. The result were as follows; 1. Anterior cruciate ligament injury:37.5% in +1 degree, 42.8% in +2 degree, 86.7% in +3 degree 2. Posterior cruciate ligament injury:12.5% in +1 degree, 13.4% in +2 degree, 33.3% in +3 degree 3. Meniscal injury:12.5% in +1 degree, 21.3% in +2 degree, 33.4% in +3 degree 4. Medial capsular ligament injury was more frequent and severe in the meniscofemoral ligament than in the meniscotibial ligament, but that finding was not correlated with degree of medial instability.