Developmental abnormalities of dens invaginatus and complications associated with their anatomy constitute a diagnostic challenge for dentists. The therapy of developmental disorders depends on the degree of anomaly, the state of the pulp and the area around periapical tissues. The treatment of dens invaginatus is necessary, also in the absence of clinical symptoms. The connection between the oral cavity and the tooth pulp resulting from invagination causes a very rapid development of inflammation, consequently leading to pulp necrosis, which may occur soon after the tooth has erupted, often before the end of root development. The aim of the study was to present endodontic treatment of dens invaginatus Oehlers type III tooth with leaving a live pulp in the main canal. Biodentine material was used for the final filling of the invagination. The study describes diagnostic methods and therapeutic recommendations for dens invaginatus.