The particularities in the clinical picture, in the diagnostic regimen and in the therapeutic schedule of eleven patients with an aneurysm showing a diameter larger than 25 mm (giant aneurysm) are presented. These aneurysms become clinically manifest as space occupation, as subarachnoid haemorrhage or TIA. 3 false diagnoses in which the aneurysm was mistaken for a cerebral tumour accentuate the necessity of a complete angiographic examination. The computer tomogram yields sufficient information permitting the suspicion of an aneurysm. In 8 patients, the direct aneurysm clipping could be performed; in 2 of them an extra-intra-cranial microanastomosis was established in the same session as a preliminary intervention. The establishment of a microanastomosis was carried out in 2 other patients. After a certain time interval, a carotid ligature (A. carotis interna) in the region of the neck was established in one of these patients. As a purely symptomatic measure for the treatment of the supratentorial pressure hydrocephalus we established a ventricular-atrial drainage in a case of basilar aneurysm. Besides 1 death and 2 severe complications, very good results could be achieved in 5 patients.