Objective: Early predictors of poor outcome after acute ischemic stroke may be useful in selecting patients for potentially beneficial but high-risk interventions. Design. Cohort study of patients given placebo in a randomized clinical trial. Setting. Multicenter trial at 139 U.S. and 14 Canadian hospitals. Patients. A cohort of 564 placebo-treated patients with major anterior circulation ischemic stroke enrolled in the Clomethiazole in Acute Stroke Study-Ischemic Stroke (CLASS-I) trial. Patients did not have significant impairment in consciousness at baseline and were enrolled within 12 hrs of symptom onset. Interventions: Prospective data collection of a number of clinical variables including use of a 6-point level of consciousness scale (1 = awake, 6 = no reaction to pain) to measure patients' level of consciousness at enrollment and 12 additional times during the first 24 hrs after enrollment. The ability of level of consciousness score and additional clinical data to predict 30-day mortality was assessed. Measurements and Main Results., At 1 month, 114 of 564 patients (20%) had died. In univariate analysis, factors significantly associated with mortality included older age, white race, higher National Institutes of Health Stroke Scale score, higher serum glucose, atrial fibrillation, and any impairment in level of consciousness (p < .05). After controlling for these factors, increasing level of consciousness score at 3 hrs after enrollment and at all but one subsequent time point was significantly associated with increased mortality (odds ratio, 1.8 per point, 95% confidence interval, 1.2-2.6, p = .003 at 3-hr time point). Maximum level of consciousness score during the initial 24 hrs of monitoring also predicted mortality (odds ratio, 1.9 per point, 95% confidence interval, 1.4-2.5, p < .001). Conclusion: The development of a decreased level of consciousness within the initial hours after stroke onset, as evaluated by a simple six-point scale, is a powerful independent predictor of mortality after major anterior circulation ischemic stroke.