To delineate the distinctive features of familial subarachnoid hemorrhage, we compared gender and age at the time of subarachnoid hemorrhage, as well as site and number of aneurysms, in patients with familial subarachnoid hemorrhage (at least 1 first-degree relative with subarachnoid hemorrhage) and patients with sporadic subarachnoid hemorrhage (no subarachnoid hemorrhage in first- or second-degree relatives), in a prospective, hospital-based series of patients. In addition we studied the pattern of inheritance in 17 families with familial subarachnoid hemorrhage. Mean age at the time of hemorrhage in patients with the familial form was 6.8 years lower than that in those with the sporadic form, and middle cerebral artery aneurysms occurred more often in patients with familial disease. Sex distribution and number of aneurysms were similar in the two groups. Inheritance was compatible with autosomal dominant transmission in some families, and with autosomal recessive or multifactorial transmission in others. In our 5 families as well as in all 18 previously reported families with two affected generations, the age at the time of subarachnoid hemorrhage was invariably lower in later generations, which is suggestive of anticipation. We conclude that familial subarachnoid hemorrhage is a separate entity with occurrence at a young age, predilection for aneurysms of the middle cerebral artery, and variable modes of inheritance, including autosomal dominant inheritance with possible anticipation.