Aim: Dens invaginatus is a malformation of the teeth, probably resulting from an infolding of the dental papilla during tooth development. The maxillary lateral incisor is the most commonly affected tooth, though not necessarily the only one. This case report shows a successful root canal treatment of a maxillary lateral incisor with this anomaly and an associated periradicular radiolucent lesion. Methods: A 12-year-old male patient with severe pain and facial swelling was referred by his general dentist for root canal treatment of the left maxillary lateral incisor. After clinical and radiographic examinations, the diagnosis resulted in an acute alveolar abscess with a periapical lesion. Radiographic examination revealed an aberrant tooth anatomy. Results: The symptoms disappeared a few days after root canal treatment was carried out and the patient's recovery was satisfactory. Regular clinical and radiographic monitoring over the following years demonstrated complete healing of the periradicular lesion. Conclusions: Accurate diagnosis of dens invaginatus led the clinician to clean, shape and fill this complex root canal system properly in order to achieve satisfactory periradicular healing without surgical intervention. Results are supported by 5 years of follow-up examination reviews. Keywords: dens invaginatus type III, "dens in dente", endodontic treatment, maxillary lateral incisor