Ischemic heart disease, which results from occlusion of one of the major coronary arteries as a consequence of thrombi and atherosclerotic plaque, continues to be the leading cause of morbidity and mortality in Western society, while stroke is the second leading cause of death worldwide. Nowadays, in addition to prevention, it is possible to treat atherosclerotic plaque by means of invasive endovascular procedures. With the advent of thrombolytic agents that favor clot lysis, treatment of patients suffering from thromboembolic diseases is greatly improved. Clots are composed of a three-dimensional fibrous network, known as fibrin gel; it is within the scaffold of this that platelets and other blood constituents get trapped, thus giving rise to the haemostatic plug. The structure of fibrin gel depends upon the polymerization conditions of fibrinogen, a glycoprotein present in the plasma of vertebrates. The thrombin-catalyzed polymerization process is usually modelled through the occurrence of a number of distinct steps that lead to the formation of fibrin monomers, which subsequently undergo polymerization to produce oligomers called protofibrils. Lateral aggregation of protofibrils forms fibers and the branching of fibers that takes place during the association of protofibrils creates the final fibrin network.The chief factor responsible for clot lysis rate is the intrinsic permeability of the fibrin network and of the individual fibers to proteolytic agents. The diffusional access from outside to proteases involved in fibrinolysis is not yet fully understood. For this reason, further knowledge of fibrin network architecture and of the packing arrangement of protofibrils would be desirable.Here we present the results of a combined Small Angle Neutron and X-ray scattering study of the packing arrangement of protofibrils. For the first time characteristic fibrils distance are related to the water trapped among fibrils and thus to space available to thrombolytic agents diffusion.