Since World War II, individual physicians and medical organizations in the United States have cooperated with the federal government in preparing for nuclear war. While most physicians have maintained a neutral stance, a minority have resisted federal policies. Health professionals participated actively at the wartime laboratories that developed the atomic bomb and in the medical research that followed Hiroshima and Nagasaki. Professional organizations helped with civil defense planning for nuclear conflict during the Cold War of the late 1950s and early 1960s. Medical resistance to nuclear war began in the same period, gained wide attention with the growth of Physicians for Social Responsibility in the early 1960s, declined during the Vietnam War, and vastly increased in the early 1980s. Activism by health professionals usually has responded to government policies that have increased the perceived risk of nuclear conflict. The recent return of civil defense planning has stimulated opposition in medical circles. Ambiguities of medical professionalism limit the scope of activism in the nuclear arena. These ambiguities concern the interplay of organized medicine and government, tensions between science and politics, and the difficulties of day-to-day work in medicine while the arms race continues.