All cases of intracranial aneurysm, arteriovenous malformation, and subarachnoid hemorrhage of undetermined etiology seen at one hospital over a 13-year period were reviewed to assess relationships between age, sex, systolic and diastolic blood pressure, and number of aneurysms. There were 350 patients, of whom 212 had aneurysms. The major findings were as follows: 1) Hypertension was not significantly more prevalent in the aneurysm population than in the age-matched general population, except for females aged 18 to 54 years (systolic pressure elevation of 10 to 15 mm Hg). 2) Under 55 years of age, both male and female hypertensive patients were twice as likely to have multiple aneurysms as normotensive patients. 3) Females were more likely than males to have multiple aneurysms. 4) For females but not males, increasing age, higher systolic pressure, and higher diastolic pressure all correlated with an increasing number of aneurysms. Hypertension appears to be more prevalent in certain subgroups of the total aneurysm population, although the individual relationships between hypertension, atherosclerosis, and aneurysms cannot be determined from either the present or previous studies. The possible role of familial factors, as well as implications for both diagnosis and further research, are briefly noted.