Background: We aimed to find the prevalence of peroneal tendon instability (PTI) accompanying different types of calcaneal fractures and to determine predictors of PTI based on preoperative CT scanning. Methods: In a retrospective cross-sectional study, preoperative CT scans of 400 consecutive calcaneal fractures undergoing surgery were reviewed for comminuted fragments in the lateral gutter of the ankle, fractures at the tip of the lateral malleolus, dislocated peroneal tendons, excessive displacement of the lateral calcaneal wall, calcaneal fracture-dislocation, superior peroneal retinaculum (SPR) avulsion fracture (fleck sign), and shape of the retromalleolar groove. The correlation of these variables with intraoperative SPR stress test, defined as the diagnostic criteria for PTI in calcaneal fractures, was evaluated. Results: In total, 369 patients (mean age, 39 ± 13; range, 11-72 years), with 321 (87.0%) of them male, were included. Among all calcaneal fractures, 67 cases (16.7%) had associated PTI as confirmed intraoperatively by an SPR stress test. A statistically significant association was found between PTI in calcaneal fractures and comminuted fragments in the lateral gutter of the ankle (P =.03), dislocated peroneal tendons (P <.001), calcaneal fracture-dislocation (P <.001), SPR avulsion fracture (P <.001), and Sanders type IV of calcaneal fracture (P =.02). There was no statistically significant relationship between PTI and the mechanism of injury (P =.98), side of fracture (P =.30), uni- or bilateral calcaneal fractures (P =.27), a fracture at the tip of lateral malleolus (P =.69), shape of the retromalleolar groove (P =.78), or excessive displacement of the lateral calcaneal wall (P =.06). The most specific CT finding to predict PTI accompanying calcaneal fractures was calcaneal fracture-dislocation (99.1%). Conclusion: Following calcaneal fracture fixation, PTI was confirmed with intraoperative SPR stress test in one-sixth of cases. With the exception of calcaneal fracture-dislocation, preoperative findings on CT scanning and calcaneal fracture pathoanatomy are insufficient to diagnose PTI accompanying calcaneal fractures. Level of Evidence: Level III, retrospective case control study.