Introduction: Persistent pain following root canal treatment (RCT), a common dental procedure, can be either of odontogenic or nonodontogenic origin. The prognosis for patients experiencing such pain is dependent on differentiating patients into these 2 categories and deriving specific diagnoses, since appropriate treatment various dramatically. This study aims to present the proportions of specific diagnoses these patients have and provide information about their signs and symptoms, including radiographic findings. Methods: This study was nested within a parent prospective observational study that followed up patients for 6 months following RCT. Patients meeting criteria for persistent pain at 6 months and living in Minnesota were considered eligible cases. Cone-beam Computed Tomography (CBCT) and dental radiographs were obtained and patients were individually evaluated by an endodontist and an orofacial pain practitioner to derive consensus diagnoses. Results: A total of 38 patients met the criteria, of which 19 were evaluated. Odontogenic reasons for persistent pain occurred less frequently than nonodontogenic reasons (42% vs. 53% respectively). There was an overlap of odontogenic and nonodontigenic reasons in 10% of the patients. About 16% reported some level of pain that seemed to be related to normal "healing". Of patients with odontogenic reasons, about one third was related to the previously treated tooth and the other two thirds were related to adjacent teeth. Temporomandibular pain disorder (TMD) was the most common nondontogenic reason for pain, comprising 42% of all patients and 80% of those with nonodontogenic pain. Persistent dento-alveolar pain disorder (PDAP) was less common, comprising 10% of all patients and 20% of those with nonodontogenic pain.Conclusion: Data from this study help in quantifying the frequency of odontogenic versus nonodontogenic reasons for persistent pain following RCT.