<p><b>OBJECTIVES</b>To evaluate the changes of the position of medulla oblongata and cerebellum following posterior fossa decompression (PFD), and to investigate their influences on the prognosis of the syringomyelia in adolescents with Chiari malformation (CM).</p><p><b>METHODS</b>A retrospective review was performed on all CM patients between September 2006 and September 2011. A subset of 46 patients, including 25 male and 21 female patients, was finally enrolled according to the inclusion criteria. The initial age and duration of follow-up averaged 13.9 years (range, 10-17 years) and 13 months (range, 6-52 months), respectively. On mid-sagittal MRI, the following parameters were evaluated pre- and postoperatively (follow-up ≥ 6 months): the longitudinal and transverse position of bulbopontine sulcus, the fourth ventricle vertex, the lower extreme of cerebella tonsil, the cervico-medullary angle, the maximal syrinx/cord(S/C) ratio and the syrinx length. Changes in these parameters were analysed using the paired samples t test, and for these reaching statistical significances, an additional bivariate correlation analysis was performed to investigate their relation with syrinx resolution.</p><p><b>RESULTS</b>At the latest follow-up, upward shifting of the bulbopontine sulcus was observed in 31 patients(67.4%), with upward shifting of the lower extreme of cerebella tonsil presenting in 35 patients(76.0%). The maximal S/C ratio and the syrinx length were significantly improved postoperatively (t = 7.114 and 7.816, P = 0.000).Significant resolution of the syrinx was demonstrated in 40 patients(86.9%), and more specifically, the average improvement rates of the maximal S/C ratio and the syrinx length were 32% ± 30%and 43% ± 33%, respectively. In addition, the bivariate correlation analysis revealed that syrinx resolution was significantly correlated with the upward shifting of the bulbopontine sulcus (r = 0.332, P = 0.027) and lower extreme of cerebella (r = 0.298, P = 0.044) .</p><p><b>CONCLUSION</b>The upward shifting of the bulbopontine sulcus and the lower extreme of cerebella tonsil might be implicated in the mechanisms of postoperative syrinx resolution.</p>