Objectives:This report presents an endodontic therapy in an unusual case of a patient who had C-shaped root canal in the mandibular first molar and had long-term follow-up radiographs. Method: A 19-year-old female patient whose right mandibular first molar was treated two weeks ago by a general dentist was referred with a history of severe pain. Radiographic examination revealed that the tooth had periapical radiolucent area together with inadequate endodontic treatment. After endodontic access preparations, gutta-percha and sealer were removed in the root canal space. The continuous C-shaped canal running from the pulp chamber to the apex was instrumented and shaped with K and H-files (VDW, Munich, Germany). Biomechanical preparations were completed with the step-back technique, and irrigating with copious amounts of 5.25% sodium hypochlorite solution. Calcium hydroxide powder (Merck, Darmstadt, Germany) was mixed with lidocaine solution and introduced into root canal system.At the second visit, the root canal system of the mandibular first molar was obturated with the UltraFil system. Results: At a recall visit 5 years later, the teeth were asymptomatic, with all findings being within normal limits. Conclusion:This unusual anatomy is more frequent in Asians than in other racial groups. Therefore the clinician must determine the anatomic configuration before commencing instrumentation. Preoperative radiographic examination and the clinical examination of the pulp chamber floor may be of aid to the root canal system.