Endothelial cell and platelet function were measured in 15 Marfan's syndrome patients and 15 age-, sex- and smoking-matched normal control subjects. The Marfan's syndrome patients had significantly increased factor VIII von Willebrand factor antigen (fVIIIvVVFAg) levels (median = 147%, inter-quartile ranges [IQRs] 117–174%) and thrombomodulin levels (median = 7.7 ng/ml, IQRs 6.1–9.1 ng/mg) compared to the control subjects (median = 82%, IQRs 64–134%), wherep= 0.007Mann-Whitney U test and (median = 2.6 ng/ml, IQRs 2.3–3.1 ng/ml),p= 0.001 Mann-Whitney U test respectively.The major complication of Marfan's syndrome is aortic dissection which is due to an inherited intrinsic defect of aortic wall structure. We postulate that the increased fVIIIvWFAg and thrombomodulin levels which were detected in the Marfan's syndrome patients are secondary to endothelial injury caused by the impaired tensile strength of blood vessels in Marfan's syndrome.