The main problem encountered with the use of biliary endoprostheses is their tendency to clog, which necessitates their exchange. We studied this problem by performing light and electron microscopy and bacterial cultures on both unused and clinically used endoprostheses. These examinations showed several basic defects in their manufacture, such as irregular inner surfaces and badly constructed side holes. The deposit that ultimately led to their occlusion was found mainly to be concentrated around the side holes. Minimizing the amount of material, bacteria, and plant fibres adhering to the endoprosthesis is the desired goal and may be achieved by improving their manufacture and design.