Background—Apoptotic microparticles are responsible for almost all tissue factor activity of the plaque lipid core. We hypothesized that elevated levels of procoagulant microparticles could also circulate in the peripheral blood of patients with recent clinical signs of plaque disruption and thrombosis. Methods and Results—We studied 39 patients with coronary heart disease, including 12 patients with stable angina and 27 patients with acute coronary syndromes (ACS), and 12 patients with noncoronary heart disease. We isolated the circulating microparticles by capture with annexin V and determined their procoagulant potential with a prothrombinase assay. The cell origins of microparticles were determined in an additional 22 patients by antigenic capture with specific antibodies. The level of procoagulant microparticles did not differ between stable angina patients and noncoronary patients (10.1±1.6 nmol/L phosphatidylserine [PS] equivalent versus 9.9±1.6 nmol/L PS equivalent, respectively). However, procoag...