Objective To evaluate the role of left atrial endocardium dysfunction and platelet activation in left atrial thrombosis (LAT), Methods Eighty mitral valve stenosis (MS) patients combined with atrial fibrillation (AF) (40 patients with LAT, 40 without), 15 healthy volunteers, and 10 left atrial appendage (LAA) specimens from donor hearts were studied.Clinical characteristics were collected before surgeries.Blood samples were collected through peripheral vein and left atrium respectively during surgeries.LAA specimens were collected during surgeries.Only peripheral blood samples were collected from healthy volunteers as controls.Expressions of vWF and P-selectin protein in LAA tissues were determined by immunohistochemistry method.Plasma concentrations of vWF and P-selectin were measured using enzyme linked immunosorbent assay method.Quantitative expression of vWF and P-selectin gene in LAA was investigated using Real-Time Fluorescent Quantitative PCR (RT-PCR).Results Compared to MS patients without LAT, patients with LAT had bigger left atrium dimension and smaller mitral valve orifice.The difference in plasma levels of vWF and P-selectin between left atrium blood and peripheral vein blood was not significant in MS patients.The peripheral plasma levels ofvWF and P-selectin in MS patients with LAT were significantly higher than those in MS patients without LAT and healthy individuals.The plasma levels of vWF and P-selectin in MS patients without LAT was higher than those in healthy individuals.VWF and P-selectin protein were stained in both left atrial endocardium and cardiomyocytes.The vWF protein stain was located near the nucleus, while the P-selectin protein stain was near the edge of cardiomyocytes.The normalized amount of vWF gene expression relative to control was 3.04 in MS patients with LAT and 2.16 in MS patients without LAT (p=0.000).The difference of p-selectin gene expression in LAA among different groups was not significant.Conclusions No difference in plasma levels of vWF and P-selectin between left atrium and peripheral vein blood was observed.The over expression of vWF gene in left atrial appendage may contribute to the increase of plasma vWF levels.vWF and P-selectin might play a collaborative role aiding to thrombosis on bases of endothelium injury in LAA.