Aim: To determine and classify the position of the mesial root canal orifices on the pulp chamber floor and examine their possible correlation with the radicular pattern in mandibular first molars. Materials and Methods In total, 110 extracted mandibular first molars were obtained and the positions of the orifices and the radicular pattern were observed using an operating microscope. Results: Of the mesial root canals, 61 (55.5%) had an independent pattern and 49 (44.5%) had a confluent radicular pattern. In canal orifices with symmetrical positions on the pulp chamber floor, the interorificial distance was key: 88% of canals were confluent at distances between 1 and 2 mm and 95.3% were independent at distances > 2 mm. In asymmetric positions, the canals had an independent pattern regardless of distance, with a percentage of 82.4%. Analysis of the independent and confluent radicular patterns showed a significant difference with the interorificial distance (P = 0.0004). Conclusion: A significant correlation was seen between the position of the mesial root canal orifices on the pulp chamber floor and an independent or confluent radicular pattern in the mandibular first molars. Keywords: dental operating microscope, mandibular molars, root canal morphology, root canal pattern