In a method and apparatus for automatic marker-free fusion (matching) of 2D fluoroscopic C-arm images with preoperative 3D images using an intraoperatively acquired 3D data record, an intraoperative 3D image is obtained using a C-arm x-ray system, image-based matching of an existing preoperative 3D image in relation to the intraoperative 3D image is undertaken, which generates a matching matrix of a tool plate attached to the C-arm system is matched in relation to a navigation system, a 2D fluoroscopic image to be matched is obtained, with the C-arm of the C-arm system in any arbitrary location, a projection matrix for matching the 2D fluoroscopic image in relation to the 3D image is obtained, and the 2D fluoroscopic image is fused (matched) with the preoperative 3D image on the basis of the matching matrix and the projection matrix.