Background and Purpose - Early vasospasm (EVSP), defined here as arterial narrowing seen on diagnostic angiography within the first 48 hours of aneurysmal rupture, is a rarely reported and poorly defined phenomenon in patients with subarachnoid hemorrhage (SAH). The purpose of this study was to characterize EVSP in a large database of such patients. Methods - We analyzed the relationship of EVSP to clinical characteristics, in- hospital complications, and outcome at 3 months among 3478 patients entered into 4 prospective, randomized, double- blind, placebo- controlled trials of tirilazad conducted in neurosurgical centers around the world between 1991 and 1997. Results - Three hundred thirty- nine (10% ) of 3478 patients had EVSP. EVSP was significantly more likely in patients with poor neurological grade on admission, history of SAH, intracerebral hematoma, larger aneurysm, thick SAH on cranial computed tomography, and intraventricular hemorrhage. EVSP was not associated with delayed cerebral vasospasm. After adjustment for differences in admission characteristics, EVSP was associated with cerebral infarction (adjusted odds ratios [ORAbstract:=1.51; 95% CI, 1.18 to 1.94; P=0.001), neurological worsening (OR=1.41; 95% CI, 1.10 to 1.81; P=0.007), and unfavorable outcome (OR=1.51; 95% CI, 1.15 to 2.00; P=0.003). In addition, there was a trend for patients with increasingly severe EVSP to have unfavorable outcome (OR=1.84 for mild and OR=2.66 for moderate/severe EVSP). Conclusions - EVSP was seen in 10% of SAH patients and was predictive of cerebral infarction and neurological worsening as well as unfavorable outcome at 3 months. EVSP was not associated with late vasospasm. EVSP may be as important as delayed vasospasm in predicting complications and long- term morbidity in SAH patients. Background and Purpose - Early vasospasm (EVSP), defined here as arterial narrowing seen on diagnostic angiography within the first 48 hours of aneurysmal rupture, is a rarely reported and poorly defined phenomenon in patients with subarachnoid hemorrhage (SAH). The purpose of this study was to characterize EVSP in a large database of such patients. Methods - We analyzed the relationship of EVSP to clinical characteristics, in hospital complications, and outcome at 3 months among 3478 patients entered into 4 prospective, randomized, double-blind, placebo- controlled trials of tirilazad conducted in neurosurgical centers around the world between 1991 and 1997. Results - Three hundred thirty- nine (10%) of 3478 patients had EVSP. EVSP was significantly more likely in patients with poor neurological grade on admission, history of SAH , intracerebral hematoma, larger aneurysm, thick SAH on cranial computed tomography, and intraventricular hemorrhage. EVSP was not associated with delayed Cerebral vasospasm. After adjustment for differences in admission characteristics, EVSP was associated with cerebral infarction (adjusted odds ratios [ORAbstract: = 1.51; 95% CI, 1.18 to 1.94; P = 0.001) , 1.10 to 1.81; P = 0.007), and unfavorable outcome (OR = 1.51; 95% CI, 1.15 to 2.00; P = 0.003). In addition, there was a trend for patients with more severe EVSP to have unfavorable outcome = 1.84 for mild and OR = 2.66 for moderate / severe EVSP). Conclusions - EVSP was seen in 10% of SAH patients and was predictive of cerebral infarction and neurological worsening as well as unfavorable outcome at 3 months. EVSP was not associated with late vasospasm. EVSP may be as important as delayed vasospasm in predicting complications and long-term morbidity in SAH patients.