We reviewed the clinical records and operative notes of seventeen consecutive patients who were treated by surgical repair for acute posterolateral rotatory instability in order to determine the diagnostic features of the instability and the effectiveness of our surgical repair in thirteen patients who returned for objective and subjective evaluation after a mean follow-up of 53.3 months. Sixteen of the seventeen knees had a positive external-rotation recurvatum test; fifteen, a positive adduction-stress test at 30 degrees of knee flexion; and twelve, a positive posterolateral-drawer test. Associated rotatory instabilities were found in ten of the seventeen knees, with anterolateral rotatory instability being the most frequent. Two patients had associated peroneal-nerve palsy. One or more components of the arcuate ligament complex were injured in all seventeen knees. None of the thirteen patients who were followed required subsequent reconstruction for any chronic instability. Of these thirteen, the results in 85 per cent were rated good subjectively and in 77 per cent, good objectively. Eighty-five per cent of these patients had returned to athletic activity at their preinjury level; the remaining 15 per cent did not participate in sports activities. A positive posterolateral-drawer test or external rotation recurvatum test, or both, was diagnostic of posterolateral rotatory instability. The adduction stress test at 30 degrees of knee flexion was usually positive, but was not diagnostic. Accurate diagnosis and treatment of posterolateral rotatory instability in the acute stage can result in subjectively and objectively acceptable knee function.