BACKGROUND: The authors conducted an in vivo study to report the incidence of fourth root canals located and treated in maxillary first molars during a seven-month period in a postgraduate endodontic program. METHODS: In this retrospective study, the authors determined the number of canals treated by postgraduate students in an endodontic program. The attending postgraduate endodontic faculty member supervising the case verified the number of canals in the teeth. The authors then collected the data from each resident and compiled them. RESULTS: The residents treated a total of 121 maxillary molars, 85 (70.2 percent) of which met the criterion of having four or more canals treated. Approximately 99 percent of the fourth canals were located in the mesiobuccal root. CONCLUSIONS: Overall, 70 percent of the maxillary first molars contained at least four canals that required instrumentation. CLINICAL IMPLICATIONS: The results of this study demonstrate that it is imperative for any dentist performing root canal therapy on maxillary first molars to examine carefully the pulpal floor to locate all canals, especially the second mesiobuccal canal. Performing thorough examinations may increase the chance of treatment success.