Chiari malformation type I (CM1) is a common neurosurgical disorder. It often causes debilitation in the affected patients. CM1 is a herniation of the caudal cerebellum into the spinal canal. This study aimed to evaluate the clinical and radiological outcomes of posterior fossa decompression and duraplasty (PFDD) in treated CM1 patients. In retrospective design, we reviewed the medical records of diagnosed patients with CM1 at two neurosurgical centers spanning 8 years from 2010 to 2017. We selected all CM1 patients who underwent PFDD surgery (n = 72) as the core sample for this study. We used the Chicago Chiari Outcome Scale (CCOS) to evaluate clinical outcomes. Pre- and postoperatively, we assessed the syrinx/cord ratio, the syrinx length, and the improvement of aqueductal stroke volume (ASV) on CSF flow MRIs. The mean value of CCOS was 14.1 +/- 2.1. On midsagittal MRIs, the mean regression in ectopia tonsils was 9.4 +/- 1.9 mm (i.e., mean pre- and postoperative tonsil herniation was 13.1 +/- 3.1 mm and 4.0 +/- 1.6 mm, respectively;p < 0.001). On coronal MRIs, the mean regression in ectopia tonsils was 8.4 +/- 1.5 mm (i.e., mean pre- and postoperative tonsil herniation was 13.9 +/- 2.4 mm and 5.8 +/- 1.0 mm, respectively;p < 0.001). A strong positive correlation was observed between clinical improvement and the increase in ASV values. CSF flow MRIs can help in the surgical decision and follow-up of CM1 patients. ASV <= 12 mu l is a significant predictor for surgical intervention. Full clinical and radiological evaluation utilizing CSF flow MRI are essential. Most syrinx cavities have regressed following PFDD.