PURPOSE: There are some criticisms of indication for internal fixation of the posterior malleolar fragments in trimalleolar fractures. We tried to find out clinical and radiologic factors which affect on a clinical outcome of trimalleolar fractures. MATERIALS AND METHODS: Thirty three patients who were treated for trimalleolar fractures and given anatomical reduction of lateral and medial malleolus were included. We divided patients into two groups, a group without the pain and the other group with the pain. Preoperative and postoperative lateral plain radiographic films were used to estimate fragment size, post-reduction gap and step off. By reviewing the medical records, other factors such as the time of ankle motion, weight loading and whether posterior malleolus was fixed. or not were stucdied. A clinical outcome was evaluated by AOFAS(American Orthopaedic Foot and Ankle Society) scaling system. We performed statistical analysis using Logistic regression analysis and Chi-square test on each factors. RESULTS: There was no definite difference between two groups on the functional outcome. There was one case showing limited ankle motion. Seven patients were involved in the group with the pain and 23 in the group without the pain. The remnant fracture gap and step off of joint surface statistically showed the meaningful corellation with the pain but a fragment size and a surgical fixation, time of motion and weight loading did not show any significances. CONCLUSION: We doubt the significance of the size of posterior malleolar fragment. We concluded that anatomical reduction of posterior malleolus is the most significant factor of a clinical outcome regardless of the size or internal fixation, especially the pain after trimalleolar injuries.