This case report presents the root canal treatment of bilateral mandibular first molars exhibiting three roots (one mesial and two distal) and five root canals (two in the mesial and three in the distal root). This unusual anatomic variation was diagnosed with an operating microscope and confirmed with the help of spiral computerised tomography (SCT). SCT was used as a non-invasive diagnostic tool to visualise the anatomy of the roots and the root canals precisely. Anatomical variations can occur in any tooth. Therefore careful examination of radiographs and internal anatomy of teeth is essential for successful root canal treatment. Keywords: dental operating microscope, mandibular first molar, spiral computerised tomography, three distal root canals