The success of root canal therapy implies a thorough knowledge of the internal as well as the external dental anatomy, in addition to its variations. An aberrant morphology of the root canals can be observed in any tooth, even in a maxillary lateral incisor. This case report looks at the root canal treatment of a maxillary lateral incisor fused with a supernumerary tooth, which shows large periapical radiolucency. This treatment was successfully carried out with the aid of computed tomography (CT) and an operative microscope. Schlagwörter: computed tomography, fused teeth, maxillary lateral incisor, operative microscope, supernumerary tooth, talon cusp