While CT scanning is useful for diagnosis of hydrocephalus and fluid-containing intracranial cysts, air studies are often necessary to determine whether the cysts communicate with the ventricular or subarachnoid space. When obstructive hydrocephalus results, decompression of the cyst is normally adequate. Hydrocephalus not caused by a cyst still requires decompression, but in addition the cyst has to be treated. Until the exact pathways of absorption of water-soluble contrast media such as metrizamide are fully understood, it may be advisable not to administer them into the ventricles in patients with obstructive hydrocephalus. A combination of CT and air studies may offer information not obtainable with either modality alone.