A colloi cyst is a cyst containing gelatinous material in the brain. It is almost always found just posterior to the foramen of Monro in the anterior aspect of the third ventricle, originating from the roof of the ventricle. Because of its location it can cause obstructive hydrocephalus and increased intracranial pressure. These cysts account for approximately 1% of all intracranial tumors. Symptoms can include headache, vertigo, memory deficits, diplopia and behavioral disturbances. The developmental origin is unclear, though they may be of endodermal origin, which would explain the mucin-producing, ciliated cell type. These cysts can be surgically resected, and opinion is divided about the advisability of this.