OBJECTIVE: Syndesmotic instability may coexist with unstable Weber B supination-external rotation (SE) lateral malleolar fractures. Current recommendations suggest that Weber B injuries should not have associated syndesmotic instability after open reduction and internal fixation of the lateral malleolus. The purpose of this study was to evaluate syndesmotic stability with respect to the current recommendations for syndesmotic fixation in Weber B SE pattern lateral malleolar fractures. DESIGN: Retrospective cohort, consecutive series. SETTING: Academic Level I trauma center. PATIENTS/PARTICIPANTS: Over a 7-year period, 238 skeletally mature patients with unstable SE pattern Weber B lateral malleolus fractures with deltoid ligament incompetence were evaluated. INTERVENTION: After lateral malleolar fixation, syndesmotic stability was evaluated by an external rotation stress examination under direct vision and fluoroscopy. MAIN OUTCOME MEASURES: The incidence of syndesmotic instability as defined by previously reported criteria. RESULTS: Syndesmotic instability was found in 93 of the 238 (39%) fractures after fixation. Instability was identified in the operating room in 92 of the 93 ankles. One case of instability was missed intraoperatively and diagnosed 2 weeks after surgery. All other patients were followed to union without displacement. CONCLUSIONS: We found syndesmotic instability to be common after anatomic and stable bony fixation in unstable Weber B SE pattern lateral malleolar fractures. Previously published criteria for syndesmotic instability based on cadaveric studies are not representative of the clinical situation. Syndesmotic instability in conjunction with unstable Weber B SE pattern lateral malleolar fractures must be sought out in the operating room with an intraoperative stress examination.