Traumatic Atlanto-Occipital (A-O) dislocation has been considered as a rare and catastrophic injury, and itsearly detection is important. But measurement method and ranges of normal data of the A-O dislocation has not beenadequatelly stressed in Korea. Therefore the authors analyzed normal ranges of A-O articulation in randomlyselected cases, prospectively. We measured the ratio of distance between BC(basion (B) and and anterior margin orposterior arch first cervical vertebra(C) and OA (opisthion (O) and posterior border of anterior arch of firstcervical vertebra (A)) in 77 cases of skull lateral tomograms and reported with reviews of the literatures. Theresults were as follows; 1. Of all cases, the largest BC/ OA ratio was 0.98. 2. The BC/OA ratio among the all agegroups were not significantly different(5% level of significancy). 3. The mean value of BC/OA ratio of all caseswas 0.77+-0.087 (SD). 4. Lateral tomogram is more helpful in visualization of the Atlanto-Occipital area thansimple roentgenogram.