Background: Internal root resorption is an endodontic disease characterized by progressive resorption of dentin from the inside of the pulp chamber. It is a comparatively rare finding in the permanent dentition, and the underlying pathology is not fully understood. Case Report: A 45-year-old patient was referred to our Department for the evaluation of the lower right canine and the upper left wisdom tooth. Pulp sensitivity tests, cone-beam tomography, and magnetic resonance imaging were used to determine the extent of lesions of the affected teeth. The teeth were subsequently extracted due the extent of the lesions. The same was the case for the upper right canine, which developed a severe internal resorption 10 months later. Micro-computed tomography of the extracted teeth revealed that all lesions had a well-defined border with no evidence of sclerosis or hypomineralization. Pulp stones were evident inside the pulp chamber. Ground sectioning of the upper right canine revealed pulp necrosis and an acute infection that had gradually moved in the apical direction. Large multi-nucleated resorbing cells were found on the dentin surface. Importantly, the apical half of the pulp exhibited comparatively normal tissue without substantial inflammatory changes. Decalcified histology of the upper left wisdom tooth demonstrated a completely different histopathological appearance characterized by chronically inflamed granulation tissue with pseudoepitheliomatous hyperplasia and massive bacterial colonization. Conclusion: Our analyses demonstrate that internal root resorption is a multifaceted dental disease with considerable variability in the rate of the underlying inflammatory changes. Oral surgeons should take this into consideration when evaluating the need for extraction of teeth with internal root resorption. Resorption of calcified tissues, such as bone, dentin, and cementum, is a prerequisite for skeletal homeostasis and tooth eruption. Resorption of permanent teeth, however, is a pathological condition that may result in the loss of the affected teeth. Tooth resorption can occur externally on the root surface or internally within the pulp chamber. External and internal tooth resorption should be considered as discrete entities which differ in terms of prevalence and etiology. External tooth resorption may affect up to 5% of all permanent teeth, and it is usually caused by trauma, ectopic teeth, or orthodontic tooth movement (1). In contrast, internal tooth resorption (IR) is a comparatively rare finding, and its etiology has not been fully elucidated (2). IR is predominantly a lesion within the upper third of the pulp chamber and may affect both the crown and the root of the tooth. The initial finding is resorption of the inner wall of the root canal that progresses in a centrifugal direction. Accompanying changes such as the resorption of the root surface or the alveolar bone may occur, but are not mandatory findings in IR. In fact, the diagnosis of IR can only be made if external factors such as root caries or cer...