Objective: In this retrospective radiographic and clinical study, we analyzed the evolution of scoliosis after posterior fossa decompression (PFD) to determine the predictors of curve progression after PFD in patients with scoliosis secondary to Chiari malformation typeⅠ (CM-Ⅰ) and syringomyelia (SM).Insttufional Review Board approval was obtained to conduct the study.Previous studies have shown that PFD can improve the associated scoliosis in some patients with CM-Ⅰ, and certain clinical factors can predict curve progression after PFD.However, the results were often contradictory.Methods: In our study, 54 patients with CM-Ⅰ who underwent a standard PFD were divided into 2 groups according to the evolution of scoliosis after PFD.One group consisted of "progressors" with curve progression of more than 5° from before PFD to the final follow-up; the other group consisted of "nonprogressors" with curve stabilization or improvement.Clinical and radiographic data were compared between progressors and nonprogressors.Results: Twenty-five of the patients received brace treatment, and 11 patients underwent spinal fusion.Nineteen patients were identified as progressors, and 35 were nonprogressors.Age, curve type, curve magnitude, and bracing treatment were independent predictors for curve progression after PFD.Both an age ≥ 10.5 years and a Cobb angle ≥ 44.5° were strongly predictive factors for curve progression.Regarding those patients with an indication for bracing, a significant difference (p=.009) was noted in terms of the evolution of scoliosis between patients with and without bracing treatment.Conclusion: PFD had an effect on the regression of scoliosis in 64.8% of the CM-Ⅰ patients.An age ≥ 10.5 years, a Cobb angle ≥ 44.5°, and double curves were factors that could predict the progression of seoliosis in these patients.Additionally, we advocate that bracing treatment may be effective in some skeletally immature patients.(C) 2013 Scoliosis Research Society.