Numerous studies have reported significant associations between ABO blood group and risk for cardiovascular disease. These studies have consistently demonstrated that thrombotic risk is significantly reduced in blood group O individuals. Nevertheless, the biological mechanisms through which ABO influences hemostasis have remained poorly understood. Exciting recent data have provided novel insights into how these ABO effects are modulated, and highlighted that ABO group significantly influences platelet plug formation at sites of vascular injury (primary hemostasis). In particular, ABO affects multiple aspects of von Willebrand factor (VWF) biology. In keeping with their reduced thrombotic risk, plasma VWF levels are approximately 25% lower in normal group O compared to non-O individuals. In addition, blood group O VWF demonstrates enhanced susceptibility to ADAMTS13 proteolysis. Finally, preliminary findings suggest that the ability of group O VWF to interact with platelets may also be reduced. Although the molecular mechanisms underlying these ABO effects on VWF have not been fully elucidated, it seems likely that they are mediated in large part by ABO(H) carbohydrate structures carried on both the N- and O-linked glycans of VWF. Interestingly, ABO(H) determinants are also expressed on a number of different platelet surface glycoprotein receptors. Recent studies support the hypothesis that ABO group not only exerts major quantitative and qualitative effects on VWF, but also may affect specific aspects of platelet function. Given the huge morbidity and mortality associated with thrombotic disorders, defining the mechanisms underlying these ABO effects is not only of scientific interest, but also of direct clinical importance.