Abstract: The method concerns automatic guiding a C-arm X-ray device equipped with a motorized exact positioning unit and essentially comprises the steps of: A) determining the position of the plane of projection (2); E) measuring the position and orientation of the reference element (3) at the bone (4) or bone joint; F) referencing the position of the plane of projection (2) to the position of the reference element (3); H) measuring the actual position and orientation of the displaced bone (4); I) determining the deviation between the actual position of the reference element (3) and the position of the reference element (3) as referenced under step F); K) determining the new position of the C-arm X-ray device (1) in order to reset the plane of projection (2) as referenced under step F); and L) guiding the C-arm X-ray device (1) in the new position as determined under step K) by means of automatic controlling means.