OBJECTIVE: Locking of the knee is usually attributed to a tear of meoiscus, or to a loose body that rnechanically obstructs the motion of joint. A less frequently recognized cause of a locked knee is a partial tear of the ACL. The purpose of this study is to report that the partial tear of,the ACL at the femoral attachment site of posterolateral bundle provided a mechanical obstruction in the knW, and it is convicted the injury mechanism of isolated ACL. MATEIRALS AND METHODS: The author presents 16 cases in which the torn ACL was observed knee locking under arthroscopy fram February 1993 to June 1996. Patients ages ranged from 22 to 54 years. The mechanism of injuries were 6 sports activity (Ski 4, Soccer 1, Badminton 1), 4 slip down, 3 pedestrian car accidents and 3 unknown causes with especially low velocity injuries. Although the chief complain in each case was knee pain, all had locking knee that, on physical examination, locked from 5 to 20 of full extension, and 4 locked full flexion. The positive Lachman test was obtained 3 of the 16 paiients. Associated intraarticular injuries were 2 tears of the medial meniscus, 1 lateral meniscus. RESULTS: The average time to arthroscopy was 3 nonths with a range of 1 month to 10 rnonths. An arthroscopic diagnosis and treatment were performed and meniscal tears were ruled out as the cause of locking in all patients. The injury mechanism of isolated ACL was the hyperextensioo of the knee with leg I/R and especially low velocity injuries. The ACL was found to have a partial tear at the site of femoral attachment of an estimated P-L bundle. The torn segment of the ACL to become interposed between the lateral femoral condyle and the latera1 tibial condyle and to act as mechanical block was demcinstrated. CONCLUSION: In all cases, the torn portion of the ACL was excised under arthroscopy and knee locking was resolved.