Multiple aneurysms of the circle of Willis developed in a 62-year-old woman with Ehlers-Danlos syndrome (EDS) within 7 months of initiation of radiation therapy to the brain. A major response of the arterial wall to radiation damage is the formation of collagen. In patients with EDS, in whom there is a defect in collagen, this repair is incomplete. We propose that the incomplete repair of the arterial wall consequent to EDS resulted in weakness of these walls, formation of an aneurysm, and, ultimately, fatal subarachnoid hemorrhage.