A cystic dilatation of cavum velum interpositum should not be confused with the diagnosis of CVI arachnoid cyst, what favors a cystic dilatation over an arachnoid cyst is that the former does not cause mass effect or hydrocephalus as it communicates with adjacent cisterns. On the opposite side, a cyst of the CVI could obstruct the ventricles, causing clinical symptoms that can mimic those of a third ventricular mass. As demonstrated in our study the ventricles are of normal size with no symptoms, and thus discovered accidentally during a routine exam of the brain.