The authors report a patient with an aneurysm of the carotid siphon who underwent ligation of the cervical carotid artery. Six years after this procedure, the patient suffered a subarachnoid hemorrhage from an apparent de novo aneurysm. Pertinent literature is reviewed to determine the incidence of this occurrence, and congenital arteriosclerotic and hemodynamic factors causing aneurysm enlargement are discussed.