Clinical trials routinely use stroke scales to compare baseline characteristics of treatment groups. It is unclear which stroke scale provides the most prognostic information. This often leads to collection of multiple scales in clinical trials. We aimed to determine which of several commonly used scales best predicted outcome. A single observer scored consecutive admissions to an acute stroke unit on the National Institutes of Health Stroke Scale (NIHSS), the Canadian Neurological Scale, and the Middle Cerebral Artery Neurological Score. Guy's prognostic score was determined from clinical data. Outcome at 2, 3, 6, and 12 months was categorized as good (alive at home) or poor (alive in care or dead). Predictive accuracy of the variables was compared by receiver operating characteristic curves and stepwise logistic regression. Of the 408 patients studied, 373 had confirmed acute stroke and completed follow-up. The three stroke rating scales each predicted 3-month outcome with an accuracy of .79 or greater. The NIHSS provided the most prognostic information: sensitivity to poor outcome, .71 (95% confidence interval [CI], .64 to .79); specificity, .90 (95% CI, .86 to .94); and overall accuracy, .83 (95% CI, .79 to .87). Logistic regression showed that the NIHSS added significantly to the predictive value of all other scores. No score added useful information to the NIHSS. A cut point of 13 on the NIHSS best predicted 3-month outcome. Baseline NIHSS best predicts 3-month outcome. The Canadian Neurological Scale and Middle Cerebral Artery Neurological Score also perform well. Baseline assessments in clinical trials only need to include a single stroke rating scale.