Background: The Thrombolysis in Myocardial Infarction (TIMI) risk score uses clinical data to predict the short-term risk of acute myocardial infarction, coronary revascularization or death from any cause. It was originally developed for use in patients with unstable angina or non-STelevation myocardial infarction. We sought to expand the clinical application of the TIMI risk score by assessing its prognostic accuracy in patients in the emergency department with potential acute coronary syndromes. Methods: We searched five electronic databases, handsearched reference lists of included studies and contacted content experts to identify articles for review. We included prospective cohort studies that validated the TIMI risk score in emergency department patients. We performed a meta-regression to determine whether a linear relation exists between TIMI risk score and the cumulative incidence of cardiac events. Results: We included 10 prospective cohort studies (with a total of 17 265 patients) in our systematic review. Data were available for meta-analysis in 8 of the 10 studies. Of patients with a score of zero, 1.8% had a cardiac event within 30 days (sensitivity 97.2%, 95% CI 96.4-97.8; specificity 25.0%, 95% CI 24.3-25.7; positive likelihood ratio 1.30, 95% CI 1.28-1.31; negative likelihood ratio 0.11, 95% CI 0.09-0.15). Meta-regression analysis revealed a strong linear relation between TIMI risk score (p < 0.001) and the cumulative incidence of cardiac events. Interpretation: Although the TIMI risk score is an effective risk stratification tool for patients in the emergency department with potential acute coronary syndromes, it should not be used as the sole means of determining patient disposition. © 2010 Canadian Medical Association or its licensors.