Unstable angina signals a sudden transition from stable to unstable atherosclerotic coronary artery disease. Although assessment of platelet activity has been shown to provide diagnostic and prognostic information, most available methods lack sufficient sensitivity or specificity for them to be useful in clinical practice. In recent years, fluorescently labeled antibodies and flow cytometry have permitted the detection of activation proteins on individual platelets. The purpose of the present study was to assess the ability of flow cytometry to detect platelet activation in patients with unstable angina and non-Q-wave myocardial infarction. Platelet activation before treatment was determined from whole-blood samples in 19 patients participating in the Thrombolysis in Myocardial Ischemia (TIMI)-IIIB study and in nine healthy volunteers (total of 24 samples) using flow cytometry and a monoclonal antibody (clone 1E3) to the activation-dependent, surface-expressed protein P-Selectin. Among healthy volunteers, a small percentage of platelets were activated (2.0 ±2.5%). In contrast, 15 out of 19 (79%) patients with suspected myocardial ischemia at rest had 10.4 ±11.1% activated platelets (95% confidence interval 4.9–15.9%; PConclusion Whole-blood flow cytometric techniques can minimize the artifactual platelet activation often seen with other techniques and therefore offer distinct advantages in the investigation of patients with thrombotic coronary syndromes. In TIMI-III, patients with unstable angina and non-Q-wave myocardial infarction had clear evidence of platelet activation. An ongoing study will examine the correlation between platelet activation assessed using flow cytometry and clinical events in a larger patient cohort.