OBJECTIVE To observe the morphology of the root canal system of the maxillary first molar mesiobuccal root using micro computed tomography and compare it to current canal classifications. METHODS Twenty mesiobuccal roots were resected from randomly selected extracted human maxillary first molar teeth and prepared for scanning using a SkyScan micro CT scanner (SkyScan 1172 X-ray microtomograph, Antwerp, Belgium) at a slice thickness of 11.6μm. Three-dimensional images were produced and analysed to record the number and configuration of the canals, the presence of accessory canals, connections between the canals and number of canal orifices and foramina. RESULTS The majority of roots had complex root canal systems with a second mesiobuccal canal present in 90% of the roots examined. Intercanal communications were observed in 55% of the roots. A single apical foramen was found in 15% of roots, two foramina were present in 20% and three or more foramina were present in 65%. In half of the roots, there were two orifices at the furcation level; 40% had one orifice and 10% had three orifices. Accessory canals were present in 85% of the roots. Only 60% of root canals could be classified using the classification of Weine et al. (1969) and 70% using the classification of Vertucci (1984). CONCLUSIONS Micro computed tomography offers an ability to examine root canal anatomy in fine detail and confirms that the morphology of the mesiobuccal root of the maxillary first molar teeth is complex and that present morphology classifications do not fully reflect the complexity.