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Introduction: The residual periapical cysts are the most common type of odontogenic cysts associated with dysplastic transformations. Therefore, considering the importance of the malignant potential of these transformations, the present study aimed to determine the extent of the dysplastic transformation of residual periapical cysts.Materials and Methods: In the present cross-sectional descriptive-analytical study, all the medical records of patients with a distinct diagnosis of residual periapical cyst over a period of 30 years (1989-2018) were extracted from the archives of the Department of Oral and Maxillofacial Pathology of Dentistry School of Isfahan University of Medical Sciences. Following that, the hematoxylin–eosin-stained slide which contained residual periapical lesions were simultaneously examined by two oral and maxillofacial pathologists for the extent of dysplastic transformations with a light microscope. The obtained results were analyzed in SPSS software (version 22) using descriptive, Chi-square, Fisher exact, and t-student tests (α=0.05).Results: The slides of 197 patients were extracted from the archive which of them, slides of 19 patients were excluded due to distortion, and finally, the slides of 178 patients were analyzed. The dysplastic changes observed were mild in 16 cases and moderate in 1 case. The frequency distribution of dysplasia was not significant based on age, gender, and anatomical location.Conclusion: In the present study, the prevalence of dysplasia in residual periapical cysts was low (about 10%); nonetheless, due to the malignant potential of dysplastic transformations, further research is recommended.