We endeavored to determine the prevalence of occipitoatlantal hypermobility in individuals with Down syndrome, to establish objective radiographic criteria for this entity, and to correlate this with neurologic abnormality. In a retrospective analysis, upper cervical spine radiographs of 210 patients with Down syndrome were compared with those of 102 normal individuals. Radiographs were evaluated using the Powers ratio. Patients identified with radiographic evidence of posterior occipitoatlantal hypermobility were then examined clinically and compared with a matched group of patients with Down syndrome and normal Powers ratios. Of the patients with Down syndrome, 8.5% had a Powers ratio of < 0.55, which was indicative of posterior occipitoatlantal hypermobility (POAH). Furthermore, 66% of those with an abnormal Powers ratio had positive neurologic findings upon physical exam, a finding that was statistically significant when compared to a matched group of patients with Down syndrome and normal Powers ratio.