Background and Purpose-Among patients with acute stroke, high blood pressure is often associated with poor outcome, although the reason is unclear. We analyzed data from the International Stroke Trial (IST) to explore the relationship between systolic blood pressure (SBP), subsequent clinical events over the next 2 weeks, and functional outcome at 6 months in patients with acute stroke. Methods-We included in the analysis 17 398 patients from IST with confirmed ischemic stroke. A single measurement of SBP was made immediately before randomization. Clinical events within 14 days of randomization were recorded: recurrent ischemic stroke, symptomatic intracranial hemorrhage, death resulting from presumed cerebral edema, fatal coronary heart disease, and death. Survival and dependency were assessed at 6 months. Outcomes were adjusted for age, sex, clinical stroke syndrome, time to randomization. consciousness level. atrial fibrillation. and treatment allocation (aspirin. unfractionated heparin. both, or neither). Results-A U-shaped relationship was found between baseline SBP and both early death and late death or dependency: early death increased by 17.9% for every 10 mm Hg below 150 min Hg (P<0.0001) and by 3.8% for every 10 min Hg above 150 mm Hg (P=0.016). The rate of recurrent ischemic stroke within 14 days increased by 4.2% for every 10-mm Hg increase in SBP (P=0.023): this association was present in both fatal and nonfatal recurrence. Death resulting from presumed cerebral edema was independently associated with high SBP (P=0.004). No relationship between symptomatic intracranial hemorrhage and SBP was seen. Low SBP was associated with a severe clinical stroke (total anterior circulation syndrome) and an excess of deaths from coronary heart disease (P=0.002). Conclusions-Both high blood pressure and low blood pressure were independent prognostic factors for poor outcome, relationships that appear to be mediated in part by increased rates of early recurrence and death resulting from presumed cerebral edema in patients with high blood pressure and increased coronary heart disease events in those with low blood pressure. The occurrence of symptomatic intracranial hemorrhage within 14 days was independent of SBP.