The balance of risks of treatment for unruptured aneurysms might change if the prognosis after rupture depends on the size of the aneurysm. In a prospective series of patients with subarachnoid hemorrhage in whom aneurysmal size was measured by CT angiography performed on admission, poor outcome occurred more often in patients with large (> or =10 mm) aneurysms (63%) than in patients with small (<10 mm) aneurysms (41%; RR = 1.5; 95% CI 1.0 to 2.2). The relative risk remained essentially the same after adjustment for age, gender, location of the aneurysm, and amount of cisternal blood.