Aim: The most common anatomy for the maxillary first molar is the presence of four root canals. However, other configurations regarding the number of root canals have been reported. The purpose of this paper is to present two cases with five root canals diagnosed during an endodontic retreatment. Materials and methods: Diagnosis was made and the endodontic retreatment was purposed and performed. A dental operating microscope was used for better understanding of this anatomy and especially looking for possible undetected canals along the developmental lines and grooves. Results: Both cases had undergone previous root canal treatments, which had failed. Both teeth had an anatomic configuration with five root canals with previously undetected MB2 and DB2, with both mesial and distal roots presenting a Vertucci Type II root canal configuration. The recall radiographs showed periapical healing. Conclusions: Although it is an uncommon configuration, the presence of both MB2 and DB2 may be present. The failure to identify the correct anatomy and locate all root canals may lead to treatment failure. The discovery of these missed canals in the endodontic retreatment of the presented cases might have been one of the reasons for retreatment success. Keywords: anatomy, molar, pulp chamber, root canal preparation, root canal therapy