We took an analysis on the clinical and radiologic findings of os odontoideum which had been regarded as relatively uncommon disease on 5 paatients whom we have recently experienced. Our results were as follows : 1. Os odontodideums were incidently discovered during the radioliogic evaluation of traumatized 4 cases who had been well being. Another one case of os odontoideum was associated with multiple epiphyseal dysplasia. 2. General radiologic findings of os odontoideum were a round or oval and hypoplastic ossicle separated from the base of odontoid process by wide gap. 3. Alanto-Axial instability was developed in 5 cases of os odontoideum by flexion and extention stree view of lateral cervical spine. 4. Flexion views of lateral cervical spine showed narrowing of anteroposterior diameter of spinal canal which were available at atlantoaxial level on 5 cases of os odontoidum, 5. CT myelogram, and cervical myelogram showed anteriorly compressed dural sac and deformed spinal cord by body of axis at the atlanto-axial level,