The present invention relates to a method for determining the position of an instrument in a structure of an object with an x-ray system, in which a 3D image data record is provided for at least one area of the object relevant for determining the position, the x-ray system is registered with the 3D image data record, after the introduction of the instrument into the structure at least one 2D x-ray image of the relevant area is recorded from at least one direction of projection with a known projection geometry with the x-ray system and a 2D position of a first location of the instrument is recorded in the 2D x-ray image. In the method a projection line in accordance with the known projection geometry is placed through the 3D image data record at the 2D position and a 3D position of the first location of the instrument in the 3D image data record is determined from an intersection of the projection line with the structure. The method enables the 3D position to be determined with a simple monoplanar x-ray system.