A retrospective clinicoroentgenographic study was done on 26 patients with atlantoaxial instability, 17 traumatic and nine nontraumatic. All were treated by means of surgical C1-C2 and occipital-C2 stabilization. The traumatic instability was associated with lesions of the odontoid process and the atlas transverse ligament. Instability may be endogenous or associated with fracture of the atlas. Surgical indication was determined by the level of the fracture line, neurologic symptoms, age, and presence of multiple lesions. C1-C2 stabilization by means of wiring and iliac graft was the selected treatment. Fusion between the occipital and C2 segment was indicated in case of irreducible dens pseudoarthrosis. Fracture on the os odontoideum was very unstable and required greater C1-C2 fusion. Nontraumatic C1-C2 instability was either congenital or secondary to pathologic fractures. Rheumatoid arthritis, which produces anterior displacement of the atlas over the dens to more than 10 mm, neurologic symptoms, or untreatable pain must be stabilized by means of C1-C2 fusion. When elevation of the dens or irreducible displacement of the atlas exists, the results were relatively poor. Tumorous instability produced pathologic fracture of the body of the axis and had to be treated with C1-C2 wiring on bone cement. Down's syndrome instability required occipitoaxial fusion and strict postoperative immobilization.