One hundred patients with Down syndrome, between one and fifteen years old, were evaluated for instability of the cervical spine at the atlanto-axial joint. Roentgenologically, the odontoid process was considered normal in ninty-seven patients, and abnormal in three patients. The mean atlanto-dens interval in ninty-seven patients with normal odontoid process was 2.5±1.0mm in flexion and 1.2±0.8mm in extension. Six patients (six per cent) exhibited abnormal intervals (4.5mm or more) during flexion. Of these patients, three patients had the slightly ligamentous atlanto-axial joint instability, another three patients who had the atlanto-axial instability with the osseous malformation of the odontoid process showed narrowing of antero-posterior diameter of the atlas and the cervical spinal canal in atlanto-axial level. One patient with abnormal odontoid process had any symptoms and signs of spinal cord compression at examination.