The invention relates to a system for carrying out a treatment of a human or animal body by a surgeon, comprising a hand-held instrument (2) having an instrument head (3) that acts on an operating field of the body and supports a treatment tool (12). The system also comprises a computer (4) on which a navigation program is provided to assist the guidance of the instrument head (3), wherein body data representing the part of the body containing the operating field, planning data representing the planned treatment, and instrument data representing the position and orientation of the instrument head (3) are available to the navigation program. A positioning means for recording the instrument data is present, wherein the navigation program compares the instrument data as actual data with the planning data as desired data, and a signalling means (10) is provided, which indicates to the surgeon a deviation of the actual data from the desired data. The positioning means comprises an image-recording means that is located at the instrument head (3) and that records, during the handling of the instrument head and in particular in rapid sequence, single images of a body part that is represented in the body data and is in a defined relationship with the treatment site, wherein the orientation of the image-recording means is in a defined relationship with the instrument head, and the navigation program generates the instrument data by matching the single images with the body data.
