In order to avoid invasive procedures (transfrontal, transcallosal) in the surgical treatment of colloid cysts the stereotactic aspiration technique was introduced by Bosch, Rähn and Backlund in 1978. The viscosity of the intracystic colloid and the displacement of the cyst away from the aspiration needle are possible reasons for unsuccessful aspiration. GT-guided stereotactic endoscopic technique gives the opportunity to fenestrate the cyst wall under direct visual control. After CT-guided stereotactic puncture of the right lateral ventricle with the foramen of Monro as target a steerable endoscope is introduced and the foramen of Monro is passed. The wall of the cyst is fenestrated and coagulated by means of monopolar (or laser) coagulation. Using this technique we have treated four patients with colloid cysts. Immediately postoperatively all of them were relieved of their complaints and have been symptom-free for a mean follow up period of twelve months. Studies of cerebrospinal fluid flow patterns, performed prior to and every six months after the endoscopic intervention, confirmed a remarkable reduction of foraminal obstruction.