STUDY DESIGN: Retrospective study of patients with scoliosis and an Arnold Chiari I malformation requiring operative management. OBJECTIVES: Determine the factors that could predict whether a particular spinal deformity might progress despite neurosurgical management of Arnold Chiari I malformation. SUMMARY OF BACKGROUND DATA: Few studies have documented the relationship between diagnosis and treatment of Arnold Chiari I malformation and associated spinal deformities. Most studies mix neural axis abnormalities and contain limited information about the spinal deformity. METHODS: Medical records, radiographs, and magnetic resonance images of patients were evaluated focusing on age and findings at presentation, characteristics of presenting and follow-up spinal deformities, and the specifics of neurosurgical and orthopedic management. Patients were divided into two groups: those whose curves progressed >10 degrees or to surgical range (largest curve >45 degrees ) after neurosurgical intervention (progressors) and those whose curves stabilized or decreased (nonprogressors). RESULTS: Eight progressors presented at an average age of 11.4 years (range 2-19) and were followed for 6.3 years (range 2-15). Seven nonprogressors presented at 6.5 years of age (range 5-10) and were followed for 6.6 years (range 3.5-14). Neurosurgical procedures were equivalent in both groups; however, surgical revisions were required in 3 out of 8 progressors and 1 out of 7 nonprogressors. All progressors had a double scoliosis curve; but only one nonprogressor had a double curve. Six out of 8 progressors and 0 out of 7 nonprogressors had a rotation >or=2+ and 50% of progressors had a thoracic kyphosis >50 degrees compared to 1 out of 7 nonprogressors. CONCLUSIONS: In this series, progression of spinal deformity after neurosurgical management of Arnold Chiari I malformation was associated with later age at neurosurgical decompressions and initial neurologic symptoms, double scoliosis curve patterns, kyphosis, rotation, and larger curve at presentation.