272 patients undergoing surgery for ankle fractures were included in this study. All patients received 5,000 U Dalteparin subcutaneously (once per day for 7 days) starting the day after surgery. Then, patients randomized to the intervention group received 5,000 U Dalteparin, and patients randomized to the control group received placebo, for 5 weeks. Patients received treatment drugs after surgery while their lower limb was either immobilized in a plaster cast or orthosis. The results following the immobilization period indicated no differences between the two groups with respect to the incidence of deep vein thrombosis (DVT). Prolonged use of Dalteparin as a method to prevent venous thromboembolism should be considered with caution.