Background: Multiple sclerosis (MS) is a chronic complex neurodegenerative disease, targeting the central nervous system (CNS) and widely believed to be autoimmune in nature. Alterations to the normal flow of Cerebrospinal fluid (CSF) in the brain could be a contributing factor to the accumulation of toxic substances in the brain interstitium and may be related to the pathogenesis of neurological disorders such as Alzheimer's disease and MS.Objective: To evaluate the CSF dynamics and its relation to MS activity using phase-contrast magnetic resonance imaging (PC-MRI).Patients and methods: This prospective study was performed between April 2020 and October 2020. A total of 45 cases were enrolled in this study divided into three equal groups: MS patients with active plaquesenhanced on MRI (group 1), MS patients with chronic plaques (group 2), and healthy controls (group 3). Quantitatively evaluation of the CSF flow was performed at the level of the cerebral aqueduct by PC-MRI.Results: Regarding to peak velocity, average velocity, net volume and average flow, there were no statistically significant differences between the three studied groups. Regarding to forward volume, reverse volume and aqueductal area, there were statistically significant differences between the three studied groups. Conclusion:The forward and reverse CSF flow volumes and aqueductal area were significantly higher in the MS patients. These findings may support the venous theory in MS patients, but may also be explained by atrophy-dependent ventricular dilatation independent of the venous theory in MS patients.