We assessed the function of the posterior malleolus, the anterior tibiofibular ligament, and the fibula with regard to posterior stability of the talus in ten ankles of cadavera. Posteriorly directed loads of as much as 200 newtons were applied. Two groups of ankles were tested; in the first group, three ankles in which the ligamentous and osseous structures were intact were tested after transection of the posterior capsule and after removal of 10, 20, 30, and 40 per cent of the articular surface of the distal end of the tibia from the posterolateral corner. In the second group, seven ankles were tested in the same sequence, but the anterior tibiofibular ligament and the fibula were transected before sectioning of the articular surface. Compared with the results for the intact ankle, the experiments on the first group demonstrated less than one millimeter of additional posterior translation of the talus after removal of as much as 40 per cent of the articular surface. In the second group, in which the anterior tibiofibular ligament and the fibula had been transected, significant posterior translation of the talus (more than three millimeters) occurred after removal of 30 per cent of the articular surface (p < 0.01). This represented a 160 per cent increase in translation compared with that in the intact ankle.