Aim: The presence of three root canals in the mesiobuccal root is an uncommon configuration in the maxillary first molar. The purpose of this paper is to present two cases that were diagnosed during an endodontic treatment. Materials and methods: Root canal therapy was performed with the aid of a dental operating microscope for better understanding of the complex anatomy. Careful analysis was given in the retreatment case, particularly looking for undetected canals that might have been a cause for endodontic failure. Results: The identification of two undetected root canals in a previously root canal therapy failure might be one of the reasons for the success of a reported case. The correct identification of an uncommon root canal configuration led to favourable results. Conclusions: The identification of unexpected anatomic configurations is mandatory. The failure to do so may lead to the failure of an endodontic therapy. Keywords: anatomy, molar, root canal therapy