Objective To study the clinical characteristics of atlanto-axial dislocation and instability caused by different reasons. Methods 388 cases of atlanto-axial dislocation and instability admitted between Jan 1975 and Apr 2000 were reviewed retrospectively. Results 262 cases were caused by anomaly, 71 by trauma and 55 by other reasons. 238/262 had bony deformity, including anomaly of odontoid process, atlanto-occipital assimilation, skull basilar invagination and some others. Simple anomaly of odontoid was the most commonly seen etiology. Myelopathy occurred at similar morbidity among each kind of patients. But patients who presented symptoms for longer time were more likely to have myelopathy. It is difficult to reduce chronic fractures of the odontoid process compared with the fresh fractures. 19 cases with chronic fracture suffered from severe myelopathy. Patients with chronic atlanto-axial fracture over 1 year were more likely to have secondary damage of spinal cord than those within 1 year. Conclusion Atlanto-axial dislocation and instability is more commonly caused by anomaly than trauma. Once the patients with atlanto-axial anomaly present clinical signs, they should be treated as soon as possible. Traumatic atlanto-axial instability should be treated at the early stage to avoid myelopathy.