This case report aimed to demonstrate the importance of cone beam computed tomography (CBCT) for the diagnosis, follow-up and treatment planning of internal root resorption (IRR). A 10-year-old male patient was referred for clinical examination, which revealed a dental fusion in the mandibular right lateral incisor (tooth 42). One year after the sectioning of the fused teeth, a well demarcated radiolucency in tooth 42 was observed by radiography, similar to IRR. CBCT scanning was performed in order to distinguish between external and internal root resorption. After the IRR diagnosis, root canal treatment was performed and no progression of the lesion was observed after 2 years. Thus, CBCT images were useful to delimit the extent of the IRR lesion, being able to identify possible communication sites with the periodontal space, and should be applied in IRR cases. Keywords: cone beam computed tomography, diagnostic radiograph, resorption