METHODS A case report. OBJECTIVE To report the case of a child with os odontoideum associated with cerebellar infarction and to discuss the correlation between atlantoaxial instability with os odontoideum and vertebrobasilar artery insufficiency. BACKGROUND Knowledge of the influence of atlantoaxial instability on vertebrobasilar artery insufficiency remains limited despite the publication of several reports. METHODS A 5-year-old boy with ataxic gait disturbance was hospitalized in the pediatric ward. Magnetic resonance imaging revealed multiple cerebellar infarctions, and cerebral angiogram showed occlusions of several branches of the basilar artery and a winding of the left vertebral artery. Stress lateral radiographs of the cervical spine showed atlantoaxial instability with os odontoideum. Posterior C1-C2 transarticular screw fixation with iliac bone graft was applied to obtain firm stability and fusion. RESULTS There was no damage to the vertebral arteries or spinal nerves in the perioperative period. Solid union of the grafted bone and rigid stability of the atlantoaxial joint were seen on lateral flexion-extension radiographs 1 year after the operation. There has been no sign of recurrent arterial insufficiency, and the patient has been free from cerebellar dysfunction to date. CONCLUSIONS Atlantoaxial instability may cause insufficiency of the vertebral artery as well as spinal cord injury. More attention should be paid to the possible relation between atlantoaxial instability and vertebrobasilar artery insufficiency.