Computer-assisted system for guiding a surgical/diagnostic instrument (27) in the body of a patient wherein a tracking sensor (20) comprises a pair of video cameras/illuminators adapted to acquire the image of a patient (P) on which a surgical/diagnostic operation is being performed by means of an instrument sensed by means of a marker visible by the sensor.The tracking sensor is movable under the thrust of actuators according to a first horizontal tilt axis and a second horizontal pan axis perpendicular to the first to move the area of view of the video cameras in space and maintain the image detected by the marker substantially at the centre of the same image also in the case of shifting of the instrument relative to the sensor (20) in order to perform a tracking function of the instrument marker by the optical tracking sensor (20).
