This report describes the case of an 82-year-old woman who presented with the chief complaint of a mild pain and gingival swelling of 2-months' duration in the buccal aspect of her mandibular left premolar region. The condition was initially diagnosed as a periodontal abscess and was treated as such by the referring clinician. The initial periapical radiograph provided by the clinician demonstrated horizontal alveolar bone resorption associated with active periodontal disease, which appeared to be more severe between the 2 premolars adjacent to the swelling. After more detailed clinical evaluation of the area and examination of the adjacent teeth, including vitality testing, it was concluded that the condition was not related to dental or periodontal disease. Consequently, a biopsy specimen was taken, and the histopathologic examination of the lesion proved it to be an odontogenic keratocyst. The case is reported because of the unusual clinical and radiographic findings associated with this lesion. Early diagnosis and appropriate treatment of odontogenic keratocyst can reduce the possibility of recurrence. However, misinterpretation of such a condition as endodontic or periodontal disease may lead to incorrect treatment planning and possibly unnecessary complications. Keywords: differential diagnosis, odontogenic keratocyst, periodontal disease, radiology