It is not possible to fully clean the walls of the root canal from the smear layer and bacteria-filled debris with a conventional chemo-mechanical root canal treatment. Therefore, various systems for the activation of irrigants are being developed, the most recent of which is lasers. The LAI method activates irrigants by creating cavitation bubbles and agitating the irrigants which reach areas difficult to access, have an antimicrobial effect and remove the smear layer. This paper describes a case of trauma of the upper left lateral incisor, with a large periapical radiolucency visible on an X-ray image. After a mechanical instrumentation with the R50 reciprocating instrument, the tooth was disinfected with the LAI technique and the Er, Cr: YSGG laser of a 2780 nm wavelength, using conical RFT2 fiber with saline. The tooth was obturated with a bioceramic sealer and the appropriate Reciproc R50 gutta-percha, while the access cavity was sealed with a glass ionomer filling (Equia Fil). The old composite filling was removed and a new one was made. A control X-ray after 6 months showed the healing of the periapical area. A suitable mechanical treatment and the disinfection of root canals are important for the success of the endodontic treatment of teeth. In the present case report, following mechanical instrumentation of the root canal of tooth #22, disinfection with Er, Cr: YSGG laser, obturation and permanent filling, the healing of the periapical area is visible after six months and the tooth is clinically asymptomatic.