A total of forty assessing doctors, mostly successful psychiatrists, were recruited to carry out the selection of the »euthanasia« victims. Upon receipt of the completed registration forms at the »euthanasia« headquarters, they were copied and the copies sent to a total of three medical experts, who, after reviewing the registration form, wrote their decision in a black box in the bottom left-hand corner of the form. A red plus sign meant the murder the patient, a blue minus the patient's survival. If their decision was not unanimous, the medical head of T4 made the final decision in his role as senior medical expert.

In the course of »Aktion T4«, more than 200,000 patients were recorded by means of the registration forms. Around 70,000 institutional patients were selected by the experts and asphyxiated with carbon monoxide in six specially set up killing centres.