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Introduction: Congenital defects of the posterior arch of atlas are uncommon anomalies that may go unnoticed. Radiographs and cone-beam computed tomography scans made for orthodontic purposes often show the atlas; therefore, these rare defects might first be noted as incidental findings on orthodontic records., Methods: A series of 7 cases of posterior arch defects of atlas (C1) that were detected during routine radiographic examination are presented here., Results: The distribution of the defects was as follows: type A, 2; type B, 1; type C, none; type D, 1; and type E, 3., Conclusion: As the potential complications associated with these defects may be quite concerning, a thorough analysis of the radiographs for these defects is imperative., (Copyright © 2020 American Association of Orthodontists. All rights reserved.)