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We report the case of a 54-year-old woman who presented at the emergency department complaining of cervical pain with no irradiation to superior extremities after a low-energy traffic accident. Physical examination was unremarkable, except for tenderness on percussion of cervical spinous process and pain with flexion and extension of the neck. A lateral cervical radiograph showed a congenital malformation (partial agenesis of the posterior arch of the atlas). Congenital defects of the posterior arch are not frequent, and their incidence is not well known. Currarino et al (Am J Neuroradiol 1994;15:249) described an anatomical classification of posterior arch defects of the atlas with 5 different types. These defects of the posterior arch of the atlas may be discovered incidentally because patients are generally asymptomatic, though chronic cervical pain and even myelopathy can occur. Treatment is normally conservative, but surgery may be indicated when patients present spinal cord compromise.