The role of haemo-dynamic factors in the development of post-stenting restenosis in coronary arteries is investigated through steady and unsteady three-dimensional (3D) numerical simulations. A simplified model of the artery, completely restored after stent implantation, is taken into account; furthermore, three simplified models of the non-completely restored coronary artery, with different percentages of residual stenosis, are investigated to study potentially hazardous blood flow patterns that may arise from the failure of the implant. The vessel wall is assumed to be rigid, while the blood is a non-Newtonian fluid, as described by the Casson model. Unsteady-state simulations are carried out with pulsatile boundary conditions, following the Womersley theory. The stent implantation increases the endothelial permeability compared to a vessel without a stent, in agreement with the literature, which can favour post-stent restenosis. Residual stenosis can expose the endothelium to hazardous conditions. The most important result of this work is a non-monotonic pattern for the endothelial permeability as a function of the degree of stenosis. In addition, the results, at a 90% degree of stenosis, indicate that the erythrocytes flow close to the endothelium downstream of the stenosis, whereas under physiological conditions, they mostly flow in the centre of the vessel.