No uniformly accepted hypothesis explains the genesis and rupture of intracranial aneurysms. We followed 5,184 men and women prospectively for 26 years; 36 cases of aneurysmal subarachnoid hemorrhage (SAH) accounted for 62% of all intracranial hemorrhages. Blood pressure before SAH was higher in these patients than in controls. Definite hypertension (greater than or equal to 160 mm Hg and/or greater than or equal to 95 mm Hg) at entry to the study or at closest exam before SAH was more frequent than in controls. Cigarette smoking, particularly heavy smoking, was also more frequent among cases.