Investigations into hemostatic abnormalities associated with sickle cell disease have been numerous. The data suggested that thrombin generation and fibrin formation were increased during steady state, with conflicting data whether further activation occurred in vaso-occlusive crisis. Platelet activation during VOC occurred, with variable findings during steady state. A selective, concomitant evaluation of the hemostatic pathways i.e. intrinsic, tissue factor (TF) or extrinsic activation, fibrinolysis, and platelet-endothelial activation had not been reported. Neither had a longitudinal evaluation been performed in infants during the unique transition period when HbF levels fall from 70 to 80 percent to less than 10 percent.