Introduction: This case report describes a mandibular second molar with four roots - three distal roots (distobuccal, middle distal, distolingual - DB, MD and DL, respectively) and one mesial root and five root canals (mesiobuccal, mesiolingual, distobuccal, middle distal, distolingual - MB, ML, DB, MD, DL) diagnosed with spiral computed tomography. Case report: A 33-year-old male reported pain in the mandibular left back tooth. On radiograph, the mandibular left second molar revealed an unusual root morphology. On spiral computed tomography, four roots and five canals were confirmed. Root canal treatment was then completed successfully. Conclusion: Although incidence of these variations is uncommon, the clinician must be aware of unusual root morphologies. Keywords: dental anatomy, distal root, endodontic treatment, mandibular second molar, spiral computed tomography