Posttraumatic chronic atlantoaxial rotatory subluxation and congenital absence of the posterior arch of the atlas are rare upper cervical spine abnormalities. The present case is that of a 4-year-old girl who had these two spinal disorders as well as spastic cerebral palsy. The interrelationship, if any, between these three conditions is unclear but presented an unusual diagnostic triad. A posterior spinal fusion between the occiput and third cervical vertebra was performed because of concern for upper cervical spine instability. The patient was immobilized in a halo vest, and a solid fusion occurred within 3 months. Preoperatively the child had never walked independently, but postoperatively, while wearing the halo vest, she was able to walk without external support, thus raising the suspicion of previous spinal instability. Fifteen months postoperatively she remains spastic but has a stable, orthotic-free, independent gait.