To analyse cerebrospinal fluid (CSF) hydrodynamics in patients with Chiari type I malformation (CM) with and without syringomyelia using 4D magnetic resonance (MR) phase contrast (PC) flow imaging. 4D-PC CSF flow data were acquired in 20 patients with CM (12 patients with presyrinx/syrinx). Characteristic 4D-CSF flow patterns were identified. Quantitative CSF flow parameters were assessed at the craniocervical junction and the cervical spinal canal and compared with healthy volunteers and between patients with and without syringomyelia. Compared with healthy volunteers, 17 CM patients showed flow abnormalities at the craniocervical junction in the form of heterogeneous flow (n = 3), anterolateral flow jets (n = 14) and flow vortex formation (n = 5), most prevalent in patients with syringomyelia. Peak flow velocities at the craniocervical junction were significantly increased in patients (-15.5 +/- 11.3 vs. -4.7 +/- 0.7 cm/s in healthy volunteers, P < 0.001). At the level of C1, maximum systolic flow was found to be significantly later in the cardiac cycle in patients (30.8 +/- 10.3 vs. 22.7 +/- 4.1%, P < 0.05). 4D-PC flow imaging allowed comprehensive analysis of CSF flow in patients with Chiari I malformation. Alterations of CSF hydrodynamics were most pronounced in patients with syringomyelia. aEuro cent Analysis of CSF flow is important in patients with Chiari I malformation aEuro cent 4D-PC MRI allows analysis of CSF in patients with Chiari I. aEuro cent Chiari I patients show characteristic qualitative and quantitative alterations of CSF flow. aEuro cent Alterations of CSF hydrodynamics are most pronounced in patients with associated syringomyelia.