Fat embolism syndrome (FES) is a rare syndrome resulting from a fat embolism, which is defined by the presence of fat globules in the pulmonary microcirculation, and it is associated with a wide range of symptoms.We aimed to assess specific unknown risk factors for FES after controlling for basic characteristics and patient's severity.and Methods: This was a nested case-control study using the Japan Trauma Data Bank database from 2004 and 2017. We included patients with FES and identified patients without FES as controls using a propensity score matching. The primary outcome was the presence of FES during a hospital stay.There were 209 (0.1%) patients with FES after trauma, and they were compared with 2,090 matched patients from 168,835 candidates for this study. Patients with FES had long bone and open fractures in their extremities more frequently than those without FES. Regarding treatments, patients with FES received bone reduction and fixation more than those without FES. Among patients who received bone reduction and fixation, time to operation was not different between the groups (P = 0.63). The overall in-hospital mortality was 5.8% in patients with FES and 3.4% in those without FES (P = 0.11). Conditional logistic regression models to identify risk factors associated with FES shows long bone and open fractures in extremities injury were associated with FES. Primary bone reduction and fixation was not independently associated with FES (Odds ratio: 1.80, 95% CI: 0.92-3.54), but delay time to the operation was associated with FES (Odds ratio: 2.21 , 95% CI: 1.16-4.23).Long bone and open fractures in injuries to the extremities were associated with FES. Although bone reduction and fixation was not associated with FES, delay time to the operation was associated with FES.