The surgical treatment of spontaneous subarachnoid hemorrhage is presented in detail. In the great majority these cases are due to the rupture of an intracranial aneurysm, located in 72 per cent of the cases in one of the principal branches of the internal carotid artery. Incidence and etiology are studied. The aneurysms are the result of incomplete em-bryological development of the arterial wall associated with a degenerative lesion. The clinical signs are absent until rupture occurs. Patient loses consciousness, goes into shock and presents involvement of some of the cranial nerves or superior centers. The presence of bloody spinal fluid and increased intracranial pressure are findings suggesting the diagnosis. Prognosis is severe since 40 per cent of the patients die in the first attack. About 30 per cent of the remaining cases expire during the second, third or fourth episodes occurring during the next ten years. The management in the acute phase is essentially conservative with absolute bed rest. After some days, arteriograms must be obtained. If the aneurysm is shown, surgical treatment must be carried out. The surgical technique is chosen after careful study of the location of the lesion, the degree of colateral circulation and the possibility of intracranial surgical attack. The indirect treatment must be combined with the direct treatment when possible. Ligation of one or more arteries in the neck and subsequent trapping of the aneurysm should be done. The different types of surgical techniques are studied. The mortality rate for conservatively treated patients is 48 per cent and 14 per cent for those treated with surgery. Five cases of intracranial aneurysms are studied and presented as illustrations of the clinical problems. These patients showed different clinical course and reflect the seriousness of the syndrome and show the necessity for prompt and adequate treatment.