The authors describe a giant intracranial internal carotid aneurysm in a patient with Marfan's syndrome. Treatment consisted of internal carotid ligation with subsequent thrombosis of the lesion. The patient's course was complicated by carotid arterial dissection and possible laminar thrombosis within the aneurysm increasing the size of the lesion. Three clinical syndromes, Marfan's, Ehlers-Danlos, and pseudoxanthoma esasticum, are discussed. The literature is reviewed with reference to the hazards of treatment of vascular pathology in patients with these connective tissue disorders.