The aim of the present study was to define the influence of basilar artery (BA) vasospasm on the outcome of patients with delayed ischemic deterioration after aneurysmal subarachnoid hemorrhage (aSAH). Sixty-five patients with clinically suspected severe cerebral vasospasm after aneurysmal subarachnoid hemorrhage (aSAH) were included in the study. All patients had angiographies done within 48 h from the initial bleeding and on the day when clinical significant vasospasm was suspected. Basilar arteries with ≥25% narrowing was found in 23 of 65 patients. Stepwise logistic regression after adjusting for age with Hunt and Hess grade, Fisher's grade, hydrocephalus and aneurysmal location as covariables revealed basilar artery narrowing â‰¥ 25% to be significantly and independently associated with unfavorable 3-month outcome (p=0.0001, OR: 10.1, 95% CI: 2.5-40.8). Basilar artery vasospasm after aSAH is an independent and significant prognostic factor associated with poor outcome in patients with clinically suspected severe cerebral vasospasm endovascular therapy.