The authors present 2 cases of tumours of third ventricle with intracranial hypertension and cerebrospinal fluid rhinorrhea. The cause of the CSF rhinorrhea most often was thinning of lamina ethmoidalis and dura mater and rarely intussusception of arachnoidea into the intrasellar space in patients with oval orifice for the stalk of pituitary body due to prolonged intracranial hypertension. Efficacious treatment of CSF rhinorrhea comprises not only removing of the tumor but also introducing the valve and in some cases tightening of the base of the anterior and middle fossa.