OBJECT: An anomalous vertebral artery (VA) position can jeopardize an otherwise successful procedure, such as a posterior cranial fossa decompression for hindbrain herniation, and may increase the propensity for VA occlusion. METHODS: The authors describe the detailed anatomy of the entrance site of the VA in adult human crania in which there is occipitalization of the atlas. They found that if the atlantal posterior arch or hemiarch was fused to the occiput one should anticipate encountering an anomalous osseous pathway as the VA enters into the cranium, as evidenced by this finding in 80% of their specimens. An anomalous entry pathway was present in all but one left-sided specimen in which the left posterior hemiarch was not fused to the occiput and one right-sided specimen in which there was an unfused and rudimentary posterior arch of the atlas. CONCLUSIONS: The clinician should consider the possibility that the VA takes anomalous routes into the skull in cases in which there is occipitalization of the atlas.