Seventy-one consecutive patients with posterolateral knee injuries had clinical stability testing abnormalities documented prospectively. We compared these findings with the incidence and patterns of their injuries documented at surgery. An abnormal reverse pivot shift test was associated with injury to the fibular collateral ligament (P = 0.01), popliteal components (P = 0.01), and midthird lateral capsular ligament (P = 0.02). An abnormal posterolateral external-rotation test at 30 degrees of flexion was associated with injury to the fibular collateral ligament (P = 0.0001) and lateral gastrocnemius tendon (P = 0.01). An abnormal adduction test at 30 degrees of flexion was associated with injury to the posterior arcuate ligament (P = 0.02). The results of this study should alert the clinician to the possibility of injury to a specific anatomic structure when the corresponding clinical stability test is abnormal. Because the fibular collateral ligament was injured in only 23% of the knees in this large series of patients, we recommend that an injury to the fibular collateral ligament not be the sole determining factor in making the diagnosis of posterolateral injuries. The wide array of injuries to many individual anatomic components that we found indicates the complexity of injuries to the posterolateral aspect of the knee.