The anterolateral and anteromedial stability of seventeen fresh frozen cadaver knees was studied in a test apparatus designed to simulate physiological conditions. Statistically significant increases in internal rotation occurred in seven knees when only the anterior cruciate ligament was sectioned, and these increases were enhanced by subsequent sectioning of the posterolateral complex and the lateral collateral ligament, singly or in combination. On the other hand, sectioning of the posterolateral complex and of the lateral collateral ligament, leaving the anterior cruciate intact, did not produce significant increases in internal rotation until the anterior cruciate ligament was sectioned in seven knees. When the entire anterolateral capsule was sectioned as far posterior as the lateral collateral ligament in three knees, no changes in internal or external rotation occurred. Only when the posterolateral complex was sectioned was there a significant increase in external rotation in any of the ligament-sectioning sequences. Thus, it appears that for pathological internal rotation of the tibia on the femur to occur, the anterior cruciate ligament must be incompetent. Clinical Relevance: The test apparatus and the results are useful in assessing which ligament structures contribute to clinically noted rotational knee instabilities.