We describe a 53-year-old Japanese woman with absence of the posterior arch of the atlas. Computed tomography revealed that the residual posterior fragment which was displaced to the right side does not correspond to a rare ossification center in the midline and that there was no clear mass of the posterior arch except for a small fragment. Moreover, the midline center was not demonstrated as a soft tissue mass. Easy tilting of the posterior fragment on flexion-extension movement suggests that the cartilaginous arch was not present between the lateral masses and the posterior fragment of the atlas. These observations support the hypothesis that hypoplastic development of the arch plays a major role in the pathogenesis of this anomaly rather than incomplete ossification. Abnormal segmentation of the vertebrae may be implicated in the etiology of this condition, since fusion of the anterior vertebral masses between the C5-6 levels, as shown in Klippel-Feil syndrome, is also observed.