Purpose: The purpose of this study is to compare the radiologic and clinical results of syndesmotic screw fixation and posterior malleolar fixation for syndesmotic injury in Lauge-Hansen classification pronation-external rotation (PER) stage IV ankle fractures with posterior malleolus fracture. Materials and Methods: We designed a retrospective study that included patients with Lauge-Hansen classification PER stage IV ankle fracture with posterior malleolus fracture. Of 723 patients who underwent ankle fracture surgery from March 2005 to November 2012, 29 were included in this study. In this study, syndesmotic injury was treated with syndesmotic screw fixation or posterior malleolus fixation. There were 15 cases of syndesmotic screw fixation and 14 cases of posterior malleolar fixation. We compared the radiologic and clinical results at one year postoperatively. Posterior malleolus fragment size on a pre-operative computed tomographic image, and tibiofibular overlap, medial clear space, articular step-off, Kellgren-Lawrence grade, and Takakura classification on a postoperative one year followup radiograph were used for comparison of the radiologic results. The clinical results were assessed using the American Orthopaedic Foot and Ankle Society score, visual analogue scale score, and patient subjective satisfaction score. Results: Posterior malleolar fragment size was $12.62%{pm}3.01%$ of the joint space in the syndesmotic screw fixation group and $27.04%{pm}4.34%$ in the posterior malleolar fixation group. A statistical difference was observed between the two groups. However, other results, including tibiofibular overlap, medial clear space, articular step-off, Kellgren-Lawrence grade, Takakura classification, and clinical scores showed no statistical difference. Conclusion: In the Lauge-Hansen classification PER stage IV ankle fracture with posterior malleolus fracture, if the posterior malleolus fracture can be reduced anatomically and fixated rigidly, syndesmotic screw fixation, which can cause several complications, is usually not required for achievement of a satisfactory syndesmotic stability; this would be a recommendable option for treatment of syndesmotic injury.