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Operating microscopes have recently been introduced to facilitate treatment of surgical and nonsurgical endodontic cases. The aim of this study was to determine in an in vivo clinical study if the use of an operating microscope would increase the number of second mesiobuccal canals located and obturated in maxillary first and second molars. Two hundred maxillary first and second molars treated with the aid of a microscope were evaluated. The number of second mesiobuccal canals that could be negotiated and obturated within 4 mm of the apex was recorded. Thirty percent of all maxillary molars had a negotiable second mesiobuccal canal. Evaluated separately, 33.1% of the maxillary first molars and 24.3% of the second molars had a negotiable second mesiobuccal canal. Results indicated that use of a surgical microscope did not increase the number of second mesiobuccal canals located, compared with those reports where access preparations were modified and the microscope was not used.