Traumatic Atlanto-Occipital (A-O) dislocation has been considered as a rare and catastrophic injury, and its early detection is important. But measurement method and ranges of normal data of the A-O dislocation has not been adequately stressed in Korea. Therefore the authors analyzed normal ranges of A-O articulation in randomly selected cases, prospectively. We measured the ratio of distance between BC(basion(B) and anterior margin or posterior arch first cervical vertebra (C) and OA(opisthion (O) and posterior border of anterior arch of first cervical vertebra (A) in 77 cases of skull lateral tomograms and reported with reviews of the literatures. The results were as follow 1. Of all cases, the largest BC/OA artio was 0.98 2. The BC/OA ratio among the all age groups were not significantly different (5% level of significancy). 3. The mean value of BC/OA ratio of all cases was 0.77$pm$0.087(SD). 4. Lateral tomogram is more helpful in visualization of the Atlanto-Occipital area than simple roent - genogram.