A 70-year old Caucasian male patient presented for the first time at the department of Operative Dentistry, Periodontology and Endodontology for routine dental control examination in November 2017. He was pain-free, had no history of trauma in the craniofacial area and was otherwise in a good to very good state of health, American Society of Anesthesiologists Physical Status Classification (ASA) []. Within the examination, radiographic bitewing images of the left and right side as well as a panoramic radiograph (Sirona Orthophos SL 3D, Dentsply Sirona Deutschland, Bensheim, Germany) were made. The image was processed using the software Sirona SIDEXIS 4 (Dentsply Sirona, Charlotte, NC, USA). Apart of the alio loco endodontically treated tooth 16 and additionally root tip resected tooth 26, the x-rays revealed a circular translucency with a cross-sectional dimension of approximately 15 to 20 mm apical to tooth 36. The translucency appeared homogenous, solid, unilocular, and with contact to the mesial root of the tooth 36. Extra-oral examination was unremarkable. During the following conversation on the radiographic finding, the patient reported that his previous long-time family dentist had already addressed this finding ten years ago. Since he assumed this osteolytic process to be an aneurysmatic bony cyst, he informed the patient that there was no treatment need. In order to receive details regarding the progression and growth of the- at that point unknown- osteolytic process, the patient was asked to provide old panoramic radiographs from the family dentist. At that appointment, sensibility of the mental nerve was tested via discrimination between sharp and blunt and two-point discrimination. The patient was able to identify the applied stimuli correctly in 100 %. The pulp sensibility of the teeth 34, 35, 36, 37 and 38 was also tested and recorded positive for the teeth tested. Indeed, at the second appointment, a panoramic radiograph made in 2011 was submitted by the patient. Here, in general, the osteolytic process looked smaller and, interestingly, the mesial root of tooth 36 did not seem to be in contact to it. In addition to the two-dimensional x-rays, a 5 x 8 cm cone beam computed tomography (CBCT, PaX-i3D, Orangedental & Co.KG, Biberach, Germany) of the left mandible was made providing important details on the topographic relationship of the radiographic lesion and other anatomic structures, such as the inferior alveolar nerve. In the region of interest, the CBCT showed a homogeneous, unilocular, translucency with a thin sclerotic margin that was not interrupted. The size of the intraosseous lesion was 16 x 12.5 x 10 mm. The continuity of the mandible seemed not to be perforated. The inferior alveolar nerve could not be detected within the lesion. The cranial margin of the lesion was directly in contact to the mesial root of tooth 36, while the distal root seemed to have a bony covering of nearly 0.5 mm. The mental foramen was located directly anterior to the lesions boundary. At this particular time, the most probable diagnosis was that of an infected radicular cyst proceeding from the mesial root of tooth 36. Differential diagnoses such as solitary bone cyst, cystic ameloblastoma or hemangioma had to be discussed as well. Based on the current CBCT, a decision was made to take a precautionary surgical tissue sample in order to obtain a distinct histopathologically verified diagnosis. Because of the close positional relationship of the intrabony lesion and the roots of tooth 36 with the resulting high risk of devitalization of this tooth, an endodontic treatment was performed before the surgical procedure. Pre-surgical endodontic treatment offers the possibility to remove potentially extruded sealer or obturation material that could potentially irritate the inferior alveolar nerve, during following surgery. Thereby, a second surgery could be avoided. The patient accepted the recommended therapy and oral and written informed consent was obtained for the endodontic-surgical-intervention.