Chiari type I malformation (CMI) patients have altered brain morphology and tissue motion compared to healthy controls. This motion may be the underlying cause of Chiari symptoms, such as occipital headaches. Highly symptomatic patients may be treated with posterior fossa decompression (PFD) surgery to restore cerebrospinal fluid flow and relieve brainstem compression. To quantitatively assess changes in brain tissue motion before and after surgery, we examined brainstem and cerebellar tissue displacement before and after surgery in 10 patients using spiral cine DENSE MRI. We found that tissue displacement in these structures in CMI patients is significantly reduced by PFD surgery.