Health care decision making in incompetent severely ill patients presents many difficult medical, ethical, and legal problems for the physician. At the same time nurses represent the group of medical professionals who have the closest contact with the patient and who have to carry out most of the decisions and instructions. Treatment decisions and their determinants have been assessed by means of a questionnaire based on three case scenarios offering three different levels of available information of the patient's wishes (no information, DNR-order, advance directive). Responses of 191 doctors and 182 nurses have been analyzed. In both groups ethical factors and patient wishes were regarded as the most important. With an increasing level of information about the patient's will the compliance increased in both groups, being more pronounced among the nurses. The more important the will of the patient was estimated, the less difficult were the decisions, the less aggressive was the treatment level chosen, the less frequent rescuscitation was made, and the more helpful the information on patient's will were experienced. These important relationships occurred mainly in the nurses. Furthermore, age and level of dementia were related with the decisions though with less importance. The more legal aspects were regarded the more aggressive treatment options were chosen. Hospital cost were less important and did not influence the decision making. The results indicate that a more frequent use of advance directives would foster patient autonomy and at the same time lessen the burden on the medical staff. A comprehensive education of both the medical professions and the general public seems warranted.