This study was designed to investigate the natural history of atlantoaxial instability in individuals with Down syndrome and to determine whether significant changes in C1-C2 relationship are taking place over time. Although more than 400 patients with Down syndrome who are presently followed at the Child Development Center had cervical spine radiographic examinations in the past, only 141 patients who had serial radiological examinations and whose radiographs were available for reevaluation participated in this study. The results of our investigations revealed that there were only minor changes (1 to 1.5 mm) of atlanto-dens interval measurements over time in 130 (92%) patients with Down syndrome. Eleven patients (8%) had changes of atlanto-dens interval measurements between 2 and 4 mm over time; however, none of these patients had any clinical symptoms. The analyses of data obtained from several subgroups (males and females, various age groups, and patients with and without atlantoaxial instability) did not show any significant changes of atlanto-dens interval measurements of successively obtained radiographs. Our recommendations for and rationale of routine screening for atlantoaxial instability and follow-up examinations are discussed in detail.