BACKGROUND CONTEXT: Eosinophilic granuloma (EG) involving the spine is uncommon in adults.Atlantoaxial instability (AAI) secondary to EG of the axis in adults is an extremely rare clinical condition that can give rise to severe neurological morbidity or mortality if not treated appropriately.There have been no previous reports on the condition in adults.PURPOSE: To present the outcome and clinical experience for the management of AAI secondary to EG of the axis in adults.STUDY DESIGN: A retrospective review study.PATIENT SAMPLE: All adult patients with AAI secondary to EG of the axis who were admitted to the spine service at the study institution between January 1999 and April 2012.OUTCOME MEASURES: Clinical symptoms, neurological status, radiological manifestations, treatment,outcome, and/or complications were recorded and analyzed.METHODS: Six consecutive adults who presented clinical and radiographic manifestations of AAI secondary to EG of axis were treated and monitored.All patients were treated surgically with anterior tumor resection and posterior reconstruction of spinal stability.Oral steroid therapy was administered after surgery as adjuvant therapy.RESULTS: The mean duration of follow-up was 77 months (range, 37-140 months).The most common radiographic feature was osteolytic destruction of the vertebral body of the axis.All patients had favorable recoveries, with osseous fusion.There were no surgery-related postoperative complications, and neither recurrence nor spinal deformity had occurred by the final follow-up examination.CONCLUSIONS: Surgical intervention via anterior tumor resection and posterior reconstruction was found to be safe and effective for treating AAI secondary to EG in adults in terms of recovering neurological function,improving symptom relief, and reducing the risks resulting from osteolytic destruction.Surgical treatment plus oral steroid therapy can produce beneficial results and defmitive local control during the follow-up period.