In acute tears of the lateral complex of the knee, it is extremely important to make a complete diagnosis not to overlook the other injured structures in the knee since tears are rarely limited to just the lateral collateral ligament. Among the primary individual structures of the lateral ligament complex that may be involved are the lateral collateral ligament, lateral capsule, popliteus muscle, arcuate ligament complex, iliotibial band, biceps femories, intermuscular septum, and of course, the lateral meniscus as well. Further, it is important to repair lateral tears as soon as possible after injury even though the lateral instability of the knee is less frequent but because it is more easily overlooked and more disabling than the other instability of the knee. Therefore, to evaluate the result of surgically treated patients having the lateral instability, we analyzed the 33 cases with minimum 1 year follow-up period who were treated at the Orthopaedic Department, Kangnam St. Mary's Hospital, from February, 1982 to June, 1985. Among 145 knee ligament injury cases, 39(26.9%) had lateral instability but six were lost to follow-up. Among these remaining 33 cases, 5(15.2%) had isolated lateral collateral ligament injury, 18(54.5%) had associated anterior cruciate ligament injury, 4(11.8%) had associated posterior cruciate ligament injury and 26(78.9%) had injury of other lateral structures including lateral collateral ligament. In all cases having associated anterior cruciate ligament injury there was severe anterolateral rotatory instability under the general anesthesia. The severity of the anterolateral rotatory instability was very much correlated with the severity of the lateral structural injuries. Eighteen out of 19 cases having associated injuries of anterior or posterior cruciate ligament, had 5 mm or more joint opening at the 0 varus stress radiogram. Thirteen(92.8%) out of fourteen isolated ligament complex injuries, and 8(53.3%) out of 15 cases having associated anterior cruciate ligament injury had good-excellent or fair(+) result. None of the patients who had associated anterior and posterior cruciate ligament injuries had good-excellet result. Therefore, the patients who had the lateral ligament complex injury associating with anterior cruciate ligament tend to have residual anterolateral rotatory instability even though the repair or reconstruction was carefully done in comparision with the patients of isolated lateral ligament complex or anterior cruciate ligament injury. Because anterior cruciate ligament injury associated with lateral ligament complex increases the anterolateral rotatory instability significantly.