Abstract: A body part such as a patient's head is positioned for treatment purposes by holding a treatment point defined relative to marks attached to the body part is held in a desired position. To this end, at least two picture signal generating sensors are aligned to observe the marks. A picture processing device for processing the pictures of the marks is provided to define marker points, which are determined by the marks and define the actual position of the body part. Furthermore, a signal processing and control device is provided for computing the actual position from the observed positions of the marker points, for comparing the actual position with a desired position and for generating actuating signals, which depend on the error between actual position and desired position. The actuating signals are applied to actuators, which engage the body part. Thereby the treatment point can be returned to the desired position, if it has deviated therefrom.