In a method for positioning a catheter that has been inserted into a vessel and a device for implementing the method, the road map technique is used, wherein as a mask image, a three-dimensional mask image of the vessel which is composed of number of individual mask images is employed. From these individual mask images, that individual mask image is selected whose exposure direction corresponds optimally to the exposure direction of an instantaneously captured individual image, in order to combine this selected individual mask image with the instantaneously captured mask image and to display the resulting combined image.