&NA;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. (Neurosurgery24:88‐92,1989)