© 2022 Elsevier Masson SASBackground: Chiari type 1 malformation (CMI) is a disorder in which cerebellar tonsils descend below the foramen magnum. Although syringomyelia associated with CMI thought to be caused by hypoplastic posterior fossa and stenosis at the craniocervical junction; it has characteristic neurological and radiological features and the exact mechanism of syringomyelia remains unknown. Purpose: The purposes of this study were to gain insight into morphological changes in posterior fossa and to find whether there is a difference in aqueductal stroke volume (ASV) between CMI with syrinx and without syrinx which may be an underlying mechanism of syrinx development. Materials and methods: We consecutively evaluated 85 patients with Chiari malformation between January 2017 and December 2019 who had undergone phase-contrast MRI examination for CSF flow and between 18–60-years-old. We divided patients into two groups as subjects with syrinx (n = 19) and without syrinx (n = 66). After evaluating morphological changes, peak and average velocity (cm/s), forward and reverse flow volume (μl), net forward flow volume (μl), ASV (aqueductal stroke volume) (μl), aqueductus Sylvi (AS) area (mm2), and prepontine cistern diameter to AS diameter ratio (PPC/AS) were calculated. Distribution of variables from two groups was evaluated by using Shapiro–Wilk normality test. Independent t test was used for groups comparison. Results: The forward and reverse volumes were statistically significantly higher in patients with syrinx (P = 0.021, P = 0.005 respectively). ASV was significantly increased in patients with syringomyelia (P = 0.014). The PPC/AS was significantly lower in patients with syrinx compared to those without (P < 0.001). AS area was significantly larger in those with syrinx. (P = 0.022). The diameter of foramen magnum was significantly lower in patients with syrinx than those without (P < 0.0001). The diameter of the herniated tonsilla at the foramen magnum level was found to be significantly lower in those with syrinx (P = 0.011). Conclusion: Foramen magnum diameter, ASV, diameter of herniated tonsil, and PPC/AS ratio are important factors in syrinx development.