The medial collateral ligament is the most commonly injured ligamentous structure of the knee and it has been demonstrated that this ligament is the prime static stabilizer of the medial side of the knee joint. The management of the medial collateral ligament injuries were divided into surgical and conservative treatment. Many investigators reported consistently good to excellent results following surgical treatment. More recently, other investigators reported that isolated medial collateral ligament injuries did equally well under non-operative as operative treatment. For accurate diagnosis, arthroscopic examination of the knee to rule out other intraarticular pathologic conditions such as cruciate ligament injuries, maniscal tears and bony injuries is essential. From October 1988 to January 1991, we treated 76 cases of isolated medial collateral ligament injuries which was confirmed by diagnostic arthroscopy. Among them, we analyzed 31 cases that minimum follow-up was over 1 year and could be re-examed, 20 cases were treated with surgery and 11 cases were not. With comparision between initial and final follow-up stress X-ray, 5.45 ± 3.17mm in non-operative group had improved in medial laxity(P<0.05). The Marshall scores which checked at final follow-up, averaged 40.3 ± 7.299 in operative and 37.364 ± 6.485 in non-operative group (P>0.05). In conclusion, medial stability was improved in operative than in non-operative group, however functional score was equally well under non-operative as operative treatment.