The prevalence of increased anterior atlanto-odontoid distance (AAOD), a risk factor for spinal cord compression, and degenerative disease of the cervical spine (DDCS) in a population of institutionalized adults with Down's syndrome (DS) was determined and compared with age- and sex-matched 'normals' presenting to a hospital emergency department. Radiographs of the cervical spines of 99 adults with DS and 198 'normals' were compared using a standardized rating scale. The prevalence of an AAOD of 3 mm or greater, the threshold of risk from the literature, was 8% for DS cases and 2% for controls (P < 0.01). The mean AAOD for DS cases was 2.0 +/- 1 mm and for controls 1.5 +/- 0.5 mm (P < 0.01). There was a negative correlation between AAOD and age of DS cases. The prevalence of any degree of DDCS among the DS cases was 64%, the controls 39% (P < 0.001); for moderate or severe DDCS the prevalence among DS cases was 45%, controls 12% (P < 0.001). The prevalence of DDCS increased with age in both groups, but the severity of DDCS was significantly increased with age in both groups, but the severity of DDCS was significantly greater for DS individuals in all age groups. The levels of the cervical spine affected ranged from C2 to C6; the most commonly affected level was C5-C6. While DS adults are at increased theoretical risk for spinal cord compression due to increased AAOD, its clinical significance would appear to be small and to decline with age.(ABSTRACT TRUNCATED AT 250 WORDS)