The enormous destructive power of present stocks of nuclear weapons poses the greatest threat to public health in human history. Technical changes in weapons design are leading to an increased emphasis on the ability to fight a nuclear war, eroding the concept of deterrence based on mutually assured destruction and increasing the risk of nuclear war. Medical planning and civil defence preparations for nuclear war have recently been increased in several countries although there is little evidence that they will be of significant value in the aftermath of a nuclear conflict. These developments have raised new ethical dilemmas for those in health professions. If there is any risk of use of weapons of mass destruction, then support for deterrence with these weapons as a policy for national or global security appears to be incompatible with basic principles of medical ethics and international law. The primary medical responsibility under such circumstances is to participate in attempts to prevent nuclear war.