The diagnosis of acute coronary syndromes is frequently missed, and many high-risk patients fail to be admitted to hospital. The aim of this study was to assess the value of cardiac markers in ruling out acute ischemic events in patients with symptoms of possible cardiac origin and nondiagnostic electrocardiograms. The data collected between May 1999 and April 2000 for this prospective cohort study were retrieved from the records of 777 consecutive prehospital patients (mean age 70 years, 62.9% men) whose symptoms lasted for 6 to 48 hours, who were treated by mobile intensive care teams, and for whom the physician could not reach a clear-cut decision whether they should be taken to hospital or left at home. The cardiac markers, creatine kinase (CK-MB), myoglobin, and troponin I, were measured at the scene using a rapid Stat kit to qualitatively detect their presence in whole blood samples. Results were determined after 15 minutes at the scene. The assay was positive in 30 patients, 11 of whom had a definite cardiac diagnosis (acute myocardial infarction in 4 and unstable angina pectoris in 7). Positive and negative predictive values of the assay for detecting a significant coronary event were 36.7% and 100%, respectively. Of the 747 patients with a negative result, 6 patients had a false result (1 with myocardial infarction and in 5 with unstable angina) (99.2% negative predictive value). Thus, cardiac markers are useful in ruling out high-risk coronary syndromes in the prehospital setting when the clinical presentation and electrocardiogram are inconclusive.