CSF ex vacuo is a proposed designation for abnormal collections of CSF replacing brain tissue which is absent or has disappeared as a result of maldevelopment, injury, infection, vascular disturbances, brain atrophy, etc. In the literature, designations such as arachnoidal, leptomeningeal and porencepalic cyst, and hydrocephalus ex vacuo, are commonly used. Even if the neuroradiological findings in CSF ex vacuo are impressive, its clinical significance may be nil or minor. However, CSF ex vacuo may develop expansive tendencies, the cause of which is not yet fully understood. On the basis of schemiatic drawings of the neuroradiological findings of eight cases of CSF ex vacuo the variations in aetiology, localization, diagnostic detectability, clinical significance and surgical implications are briefly discussed.