A method and apparatus for introducing a stent into a region of a major blood vessel within the human body in a manner which substantially reduces the risk of embolic material escaping to the vessel and causing a blockage at a downstream location. Additionally, the apparatus includes a uniquely designed stent delivery catheter having a central lumen through which a guide wire travels, the central lumen being provided with a valve means that is operable by the guide wire and when moved into a closed position by withdrawal of the guide wire, functions to prevent the flow of loose debris toward the proximal end of the catheter.