CINE phase-contrast MRI (CINE-MRI) was used to measure cerebrospinal fluid (CSF) velocities and flow rates in the brain of six normal subjects and five patients with communicating hydrocephalus. Mathematical brain models were created using the MRI images of normal subjects and hydrocephalic patients. In our model, the effect of pulsatile vascular expansion is responsible for pulsatile CSF flow between the cranial and the spinal subarachnoidal spaces. Simulation results include intracranial pressure gradients, solid stresses and strains, and fluid velocities throughout the cranio-spinal system. Computed velocities agree closely with our in vivo CINE-MRI CSF flow measurements. In addition to normal intracranial dynamics, our model captures the transition to acute communicating hydrocephalus. By increasing the value for reabsorption resistance in the subarachnoid villi, our model predicts that the poroelastic parenchyma matrix will be drained and the ventricles enlarge despite small transmantle pressure gradients during the transitional phase. The poroelastic simulation thus provides a plausible explanation on how reabsorption changes could be responsible for enlargement of the ventricles without large transmantle pressure gradients.