ABO histo-blood group is a major determinant of plasma levels of factor VIII (FVIII) and von Willebrand factor (vWF). Blood group O individuals have significantly (approximately 25%) lower plasma levels of both glycoproteins. This association is of clinical significance. Low plasma levels of either FVIII or vWF have long been established as causes of excess bleeding. Conversely, there is accumulating evidence that elevated FVIII-vWF levels may represent an important risk factor for ischaemic heart disease and venous thromboembolic disease. In spite of the well-documented association between ABO blood group and FVIII-vWF levels, the underlying mechanism remains unknown. However, it has been established that the ABO effect is primarily mediated through a direct functional effect of the ABO locus on plasma vWF levels. Theoretically, ABO blood group may alter the rate of vWF synthesis or secretion within endothelial cells. Alternatively, ABO group may affect vWF plasma clearance rates. ABH antigenic determinants have been identified on the N-linked oligosaccharide chains of circulating vWF and FVIII, according to the blood group of the individual. It remains unclear whether these carbohydrate structures are responsible for mediating the effect of ABO blood group on plasma vWF levels.