The MRI findings in a 6-year-old boy with an astrocytoma of the mesancephalon are reported. A ventriculocisternostomy had been performed in order to reduce the hydrocephalus. At the site of the ventriculocisternostomy, the T2-weighted images showed a low signal in the anterior part of the third ventricle, the interpeduncular and the pontine cistern. This was attributed to CSF flow void. We conclude that MRI can provide information about the precise location and normal functioning of a ventriculocisternostomy.