THE FAMILIAL OCCURRENCE of cerebral aneurysms in a defined clinical group of 1150 patients from a defined catchment area was studied. Two or more proven aneurysmal subarachnoid hemorrhage (SAH) patients within the same family were included. Of these 1150 patients, 113 (10%) had a proven familial occurrence of aneurysmal SAH. These 113 patients form 91 SAH families with a total of 203 aneurysm patients. Fifty-four percent of the patients were female, and the mean age in female patients was 49 years and in male patients was 44 years. In 23% of the families, three or more members were identified as having cerebral aneurysms. Middle cerebral artery aneurysms were the most common type (47%). Based on the high incidence (10%) of familial intracranial aneurysms among SAH patients, a prospective study of healthy family members for incidental intracranial aneurysms was performed, with positive findings of 12%.