Objective: To evaluate the effect of an intervention on the understanding and use of DNR orders by physicians; to assess the impact of understanding the importance of involving competent patients in DNR decisions. Design: Prospective clinical interventional study. Setting: Internal medicine department (70 beds) of the hospital of La Chaux-de-Fonds, Switzerland. Participants: Nine junior physicians in postgraduate training. Intervention: Information on the ethics of DNR and implementation of new DNR orders. Measurements and main results: Accurate understanding, interpretation, and use of DNR orders, especially with respect to the patients' involvement in the decision were measured. Junior doctors writing DNR orders had an extremely poor understanding of what DNR orders mean. The correct understanding of the definition of a DNR order increased from 31 to 93% (p<0.01) after the intervention and the patients' involvement went from 17% to 48% (p<0.01). Physicians estimated that 75% of their DNR patients were mentally competent at the time of the decision. Conclusion: An intervention aimed at explaining the ethical principles and the definition of DNR orders improves understanding of them, and their implementation, as well as patient participation. Specific efforts are needed to increase the involvement of mentally competent patients in the decision.