1. A method for navigation-calibrating x-ray image data, comprising the following steps: an x-ray image is produced without the patient by means of an x-ray device into the radiation path of which a calibration instrument (C) is introduced which can be positionally detected and tracked in a medical navigation system; said x-ray image without the patient is registered in said navigation system with the aid of localization structures ( 18, 19 ) arranged on at least two spaced supports ( 3, 4 ) of said calibration instrument (C), and images ( 18 ′, 19 ′) of the same on said first x-ray image; a support ( 4 ), together with its localization structures ( 19 ), is removed from said calibration instrument (C); a patient x-ray image is produced; the calibration information from said images ( 18 ′) of said localization structures ( 18 ) of the remaining support ( 3 ) on said instrument (C) are compared for the two x-ray images, and calibration is corrected if there is an insufficient correspondence between said images ( 18 ′).