The aim of this paper is to present the exceptional case of a lipoma located beneath the floor of the third ventricle in front of right mamillary body manifesting with psychiatric symptoms and treated by navigated neuroendoscopy. A 58-year-old male was referred for neurosurgical treatment because of paroxysmal disturbances of consciousness accompanied by aggressive behaviour. Because electroencephalographic evaluation failed to confirm the epileptic origin of the presenting symptoms, the final psychiatric diagnosis was syndrome of amentia. However, when the MRI investigation revealed two tiny lipomas with one of them located in front of the right mamillary body, a hypothesis was established that this lipoma might be the cause of the clinical symptoms. After a thorough analysis of the patient's symptoms and taking into special consideration the functional role of hypothalamic structures the decision to remove the lipoma endoscopically was accepted.