Twelve cadaveric knees were tested to determine effective reconstructive treatment for severe chronic posterolateral rotatory knee instability accompanied by excessive varus and posterior laxity. Posterolateral, varus, and posterior laxity were measured, first with the ligaments intact, then after complete sectioning of the posterior cruciate ligament (PCL) and posterolateral structures, and finally after reconstruction of these structures in different orders. The increases in those laxities were produced following the sectioning of all of the structures and disappeared throughout the flexion range after combined reconstruction of the PCL, lateral collateral ligament (LCL), and popliteus tendon. However, some residual increase in the laxity was always observed if any of the three structures were excluded from reconstruction. Therefore, combined reconstruction of the PCL, LCL, and popliteus tendon is essential and adequate for treating severe chronic posterolateral rotatory instability.