Jehovah's witnesses refuse transfusion of blood and blood products on the basis of religious convictions even when transfusion is considered necessary to save life. Medical treatment of these patients presents an ethical challenge for the physicians. The legal aspects of either administering blood to or withholding necessary blood transfusion from a Jehovah's witness are not clarified. Competent adult patients cannot be treated against their will. Children and unconscious patients must be treated according to "jus necessitatis". Whether a competent advance directive from a patient before an operation in general anaesthesia can be or must be respected is legally obscure. Final clarification of the patients' rights and the physicians' legal status could be obtained by an amendment to the existing Practice of Medicine Acts as proposed in the report (1184) from the Danish Ministry of Justice. The ethical aspects of administering blood to or withholding blood from these patients are also complex. It is recommended to determine one's own attitude individually and to inform the patient about this before an operation.