BACKGROUND Patients with obesity who undergo surgery have an increased risk for perioperative complications. The purpose of this study was to determine the perioperative outcome of rotationally unstable pelvic fractures treated with initial external fixation in obese patients. METHODS Review of clinical data and radiographs of 42 patients identified from a prospectively entered trauma database who had rotationally unstable fractures of the pelvis treated with initial pelvic external fixation. RESULTS Inability of an anterior uniplanar external fixator to maintain adequate reduction of the pelvic ring disruption occurred with a greater frequency in obese patients compared with nonobese patients (p < 0.005). The anterior frame was unable to provide sufficient stabilization of the pelvic ring disruption in all obese patients with pure open-book-type fractures. For these patients, stabilization of the pelvic ring was achieved with early symphyseal plating. CONCLUSIONS We report a significantly higher incidence of inability to obtain or maintain reduction of open-book pelvic fractures in obese patients using primary anterior uniplanar external fixation. Early symphyseal plating allowed early patient mobilization and maintained acceptable reduction throughout the follow-up period.