Objective Recent studies reveal important roles of platelet P-selectin on progression of atherosclerosis. In the present study, we examine the relation between platelet P-selectin expression and severity of acute coronary syndromes. Methods One hundred and eighty-four consecutive patients with proven or clinically suspected acute coronary syndromes (ACS) were enrolled in the study. Level of P-selectin expression was determined by flow cytometry. Platelet P-selectin level was expressed as the percentage of Pselectin positive platelet. Results The level of P-selectin was higher in patients with a single diseased coronary artery or multiple diseased arteries compared to those with normal coronary arteries. P-selectin expression was significantly and positively correlated with angiographic Gensini score (r=0.323, P=0.029). Multiple regression analyses showed that the association of the percentage of Pselectin–positive platelets with ACS was independent of other clinical factors. Conclusions Platelet P-selectin is associated with severity of acute coronary syndromes in patients with acute coronary syndromes.