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. A uniquely designed stent delivery catheter includes a central lumen through which a guide wire travels. The central lumen is 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.