A 21-year-old man developed signs of brainstem damage after being injured while playing rugby. Cervical x-ray films showed os odontoideum, and angiography revealed persistent occlusion of the right vertebral artery at the level of the second cervical spine. These findings indicated that atlantoaxial dislocation caused by os odontoideum may have induced vertebral artery occlusion, leading to brainstem infarction.