1. A method of determining the position of a medical instrument ( 13 ), partly introduced into an object ( 3 ) to be examined, in a three-dimensional image data set (CT) of the object ( 3 ) to be examined, which method includes the following steps: acquisition of a two-dimensional X-ray image (I r ) of the object ( 3 ) to be examined by means of an X-ray device ( 2 ), determination of the spatial positions of the X-ray image (I r ) and the medical instrument ( 16 ), determination of the spatial correlation between the X-ray image (I r ) and the three-dimensional image data set (CT), and determination of the position of the medical instrument ( 16 ) in the three-dimensional image data set (CT) from the spatial position of the medical instrument ( 16 ) on the basis of the spatial correlation between the X-ray image (I r ) and the three-dimensional image data set (CT).