In a method and apparatus for the automatic merging of 2D fluoroscopic C-arm images with preoperative 3D images with a one-time use of navigation markers, markers in a marker-containing preoperative 3D image are registered relative to a navigation system, a tool plate fixed on the C-arm system is registered in a reference position relative to the navigation system, a 2D C-arm image (2D fluoroscopic image) that contains the image of at least a medical instrument is obtained in an arbitrary C-arm position, a projection matrix for a 2D-3D merge is determined on the basis of the tool plate and the reference position relative to the navigation system, and the 2D fluoroscopic image is superimposed with the 3D image on the basis of the projection matrix.