Intoduction: External invasive root resorption (EIRR) is an insidious and often aggressively destructive form of external root resorption, that occurs when the protective precementum layer is mechanically damaged or removed which allows for the denuded areas of the root surface to be colonized by clastic cells and for the resorption to get progressively worse, which usually affects a single tooth. This type of resorption develops slowly and initially without symptoms and may occur as a late complication following dental trauma particularly where it involves damage to cementum and supporting tissues. While this resorption may be evident clinically as a pink coronal discolouration, later with enamel's cavitation, often there are no obvious external signs and the condition is occasional detected radiographically. It is characterised by the invasion of the cervical region of the root by fibrovascular tissue, which progressively resorbs dentine, enamel and cementum. The dental pulp remains protected by an intact layer of dentine and predentine until late in the process. Ectopic calcifications can be observed in advanced lesions both within the invading fibrous tissue and deposited directly onto the resorbed dentine surface. The aetiology of external invasive root resorption is unknown but trauma has been documented as a potential predisposing factor (mostly concussion), however orthodontic treatment, orthognathic and dentoalveolar surgery, periodontal therapy and tooth whitening procedure can also induce external invasive root resorption. Objectives: As early diagnosis is difficult, the dentist should be aware of this pathology and severity of clinical signs that may suggest it's presence, to be self sufficient and capable of diagnosing the aforementioned dental pathology. Methods and results: Male patient, 45 years, emerged at Dental Clinic of Faculty of Medicine, University of Coimbra, Portugal, with symptoms to percussion in tooth 15, right second premolar, restored with ceramic onlay held for nearly five years without leakage, no hemorrhage on probing or periodontal pocket and by conducting periapical ortorradial observed endodontic treatment accompanied by a radiolucent area in the demarcated middle third root structure by a thin radio-opaque line that matches the outlines of the canal area overlapping the irregular external invasive root resorption which may be associated with a slight irregularity in the adjacent bonecrest. On the first moment, endodontic treatment was performed; medication intracanalar was calcium hydroxide (Calcicur-Voco) and thermoplastic obturation (SystemB-Analytic Technology) was made aided with gutta percha and endodontic sealer based on epoxy resin (AHPlus-Dentsply). After 18 months, the patient appeared again with new pain symptoms, accompanied by edema and erythema of the buccal and palatal region of tooth 15. Endodontic surgery was performed with curettage of the lesion and lateral periodontal bone regeneration was made with natural bone substitute material (Bio-Oss-Geistlich). Conclusion: Controls were affected after 12 and 30 months, at the time of controls and nowadays the tooth remains asymptomatic and functional. Keywords: external invasive root resorption, MTA, diagnosis, treatment