The first mandibular molar shows a complex range of anatomical variations concerning the number of roots and root canals. The mesial root usually has two canals, whereas the distal root mostly has one canal. Only 5 cases presenting four canals in the mesial root of a mandibular molar were reported. However, the occurrence of a distal root with three canals in a mandibular molar with six canals was rarely described in scientific literature. The main objective of these case reports is to draw attention to the clinical challenges of possible complex anatomic configurations in canals of mandibular first molars. Parole chiave: complex root canal pattern, dental operating microscope, mandibular first molar, root canal morphology