Objective: To determine the association between admission blood glucose and outcome in ischemic stroke patients treated with thrombolysis. Design: A prospective, open, multinational, observational study. Setting: An ongoing Internet-based, academic-driven, interactive thrombolysis register. Patients: Between 2002 and 2007, 16 049 patients were recorded in the SITS-ISTR. Main Outcome Measure: Blood glucose was recorded at admission. Blood glucose was divided into the following categories: less than 80, 80-120 (reference range), 121-140, 141-160, 161-180, 181-200, and greater than 200 mg/dL. Outcomes were mortality and independence (modified Rankin Scale score of 0-2) at 3 months and symptomatic intracerebral hemorrhage (SICH) (National Institutes of Health Stroke Scale deterioration ≥4 points within 24 hours and type 2 parenchymal hemorrhage). Results: In multivariable analysis, blood glucose as a continuous variable was independently associated with a higher mortality (P < .001), lower independence (P < .001), and an increased risk of SICH (P = .005). Blood glucose greater than 120 mg/dL as a categorical variable was associated with a significantly higher odds for mortality (odds ratio [OR], 1.24; 95% confidence interval [CI], 1.07-1.44; P=.004) and a lower odds for independence (OR, 0.58; 95% CI, 0.48-0.70; P < .001), and blood glucose from 181 to 200 mg/dL was associated with an increased risk of SICH (OR, 2.86; 95% CI, 1.69-4.83; P < .001) compared with the reference level. The trends of associations between blood glucose and outcomes were similar in patients with diabetes (17%) or without such history, except for mortality (P = .23) and SICH (P = .06) in which the association was not statistically significant in patients with diabetes. Conclusions: Admission hyperglycemia was an independent predictor for poor outcome after stroke/thrombolysis, though SICH rates did not increase significantly until reaching 180 mg/dL. These results suggest that tight control of blood glucose may be indicated in the hyperacute phase following thrombolysis. Randomized trial data are needed. ©2010 American Medical Association. All rights reserved.