The aim of this study was to retrospectively determine the prevalence of severe external root resorption (Score 4 according to Malmgren et al.), in maxillary incisors, during fixed orthodontic treatment and to evaluate the possible predisposing factors for root resorption. Material and Methods: The treatment records of 7000 patients who have been treated between years 1990 and 2019 at the Department of Orthodontics Faculty of Dentistry Marmara University were examined to determine the prevalence and the predisposing factors of external root resorption. The following data was retrieved from the patients' records: age at the beginning of the treatment, gender, root morphology, overjet, overbite, treatment modality (extraction, non-extraction), treatment duration, buccal and palatal alveolar bone thickness for the maxillary incisors, and amount of movement of the incisal root apices and incisal edges. The relationship between root resorption and the predisposing factors was assessed using chi square and independent t-tests. Results: The prevalence of severe external root resorption was 3.23%, and the results have demonstrated significant difference between the groups for the variables: treatment modality (extractions), increased treatment duration, cortical thickness of the alveolar bone, and amount of incisor movement at the end of the treatment. Conclusion: It can be concluded that extractions, increased treatment duration, thin alveolar bone, and excessive incisor movement represent risk factors for severe root resorption in maxillary incisors following orthodontic treatment.