Introduction Description of syrinx resolution after posterior fossa decompression (PFD) in patients with scoliosis secondary to Chiari malformation type Ⅰ (CMI) and syringomyelia (SM) has been rarely reported in the literature.This study was performed to investigate the outcome of PFD in patients with scoliosis secondary to CMI and to identify potential predictive factors for better outcome after PFD.Material and methods Patients with scoliosis secondary to CMI and SM, who had undergone PFD during the period 2000 through 2009, were recruited.Inclusion criteria were (1) age ≤ 18 years, (2) diagnosis of SM associated with CMI, (3) scoliosis as the first complaint, (4) having undergone preoperative and follow-up magnetic resonance imaging (MRI).Patients with acquired CMI anomalies or who had received syringosubarachaoid shunting were excluded.The maximal S/C ratio and syrinx length were measured to evaluate syrinx resolution after PFD.A 20% decrease in S/C ratio or length at the latest follow-up was defined as a significant radiographic improvement and complete resolution was used to describe the syrinx disappearing after PFD.Results 44 patients were recruited.Follow-up MRI was conducted for all 44 patients at 6 ± 3 months postoperatively, for 37 patients at 2 years ± 3 months, for 26 patients at 4 years ±3 months, and for 15 patients at 6 years ± 3 months.97.7% (43 of 44) of patients showed significant radiographic improvement by MRI.The distance of tonsillar descent (mm) was correlated significantly with the surgical outcome (r =0.116, P =0.013).Significant improvement was observed within 6 months postoperatively, with continued slow improvement after that.Conclusion Syringes showed significant improvement after PFD in most patients with seoliosis secondary to CMI.Resolution generally occurred within 6 months follow-up and continued at a slow rate for several years.In addition, the severity of tonsillar descent is a potential predictor for better improvement after standard PFD.