1. A method for ascertaining the position of a structure in a body, comprising the method steps of: providing 3D data which represents a three-dimensional image of at least the part of the body which contains the structure; providing at least two 2D image data sets which each represent an x-ray image of a part of the body containing at least the structure, from a known viewing direction; generating virtual x-ray images from the 3D data for a virtual position of the 3D data, wherein for each viewing direction for which a 2D image data set has been provided, a corresponding virtual x-ray image is generated; comparing the virtual x-ray images in pairs with the corresponding 2D image data sets and generating a cumulative degree of similarity for the virtual is position of the 3D data from all the comparisons; repeating the steps of generating virtual x-ray images, comparing in pairs and generating a cumulative degree of similarity for different virtual positions of the 3D data; and ascertaining the position of the structure in the body from the virtual position of the 3D data which results in the greatest cumulative degree of similarity.