The former Soviet Union developed variola virus, which causes smallpox, for use as a biologic weapon, and supplies may have fallen into other hands. As Lev Sandakhchiyev, the director of Russia's Vektor Institute, has warned, “All you need is a sick fanatic to get to a populated place.”1 U.S. experts agree.2,3 The benefits of preemptive, voluntary vaccination are great. One immediate outcome is deterrence. Vaccination before exposure dramatically reduces the value of smallpox as a weapon; in the case of an attack, the number of unimmunized persons will be greatly reduced, and the outbreak will be much easier to manage. The United States is the most likely target of bioterrorism, and preexposure vaccination in this country reduces the risk of secondary exposure elsewhere in the world. A survey has shown that 61 percent of Americans would want to be vaccinated if smallpox vaccine were available, and the public thus appears to be ready for this approach.4