The atlas-dens interval (ADI) was measured on lateral cervical spine roentgenograms in 200 normal children, ages 3 to 15 years. One hundred examinations were done at a 40 in. tube to film distance with the child supine, and another 100 at 72 in. tube to film distance with the child sitting. Flexion, neutral and extension roent- genograms were obtained and analyzed. At the 40 in. tube to film distance with the patient supine and the head in neutral position, no child in the series had an ADI greater than 4 mm. Therefore, a measurement greater than 4 mm. should alert one to the possibility of atlanto-axial subluxation. At the 72 in. tube to film distance with the patient erect, there was a 5 mm. ADI in a normal 13 yr. old boy. This was the maximum normal ADI at this distance. Age and sex are not statistically significant factors in evaluating the ADI; however, position can influence the measurements. The neutral position is recommended for measurement. The mean difference in measurements between readers was 0. 25 mm. in neutral position. Other signs of atlanto-axial subluxation are: increased soft tissue anterior to cervical spine; greater than 10 degree flexion between the atlas and axis; compensatory curve of the lower cervical spine; and narrowing of the atlan-to-vertebral foramen.