The clinician's difficulty in performing the correct diagnosis for some endodontic situations sometimes leads to a compromised treatment of some teeth. Considering this, a consistent knowledge of the root canal morphology and its variations is very important to the success of root canal treatment. This case report presents the retreatment of a mandibular first molar with four root canals, of which three were located in the mesial root. A middle mesial root canal was found between the mesiobuccal and mesiolingual root canals with the use of magnification and ultrasonic tips. The morphological pattern of separate apical terminations of the three mesial root canals, as manifested in this case, is a very rare condition. After 1 year, the tooth revealed adequate clinical function, and radiographic findings showed adequate healing. It can be concluded that the knowledge of the root canal anatomy is important to prevent endodontic failures. Keywords: endodontic retreatment, middle mesial canal, root canal anatomy