We took an anlysis on the clinical and radiologic findings of os odontoideum, which had been regarded as relatively uncommon disease, on 5 patients whom we have recently experienced. Our results were as follows: 1. Osodontoideums were incidently discovered during the radiologic evaluation of traumatized 4 cases, who had been wellbeing. Another one case of os odentoideum 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 stress 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 myelogaram showed anteriorly compressed dural sac and deformed spinal cord by body ofaxis at the atlanto-axial level.