The authors describe a 12-year-old boy with dyskinetic (athetoid-dystonic subtype) cerebral palsy and os odontoideum. Dystonic and choreoathetotic components in cerebral palsy are movement disorders that are difficult to treat and cause major disability. Dystonic posturing causes excessive flexion, extension, and rotation of the neck. Repetitive abnormal movements in patients with this type of cerebral palsy give rise to a higher incidence of pathologic conditions affecting the craniovertebral junction. Os odontoideum is one of these pathologies, and it represents a rare anomaly of the odontoid process. There are only a few reports describing os odontoideum in children with dyskinetic cerebral palsy. This clinical and neuroradiologic study focuses on the problem of atlantoaxial instability and os odontoideum in these forms of cerebral palsy, which is too often underestimated.