What can be achieved by the vaccination of individuals exposed to smallpox virus after release of the virus by bioterrorists? There exist several past sources of information on postexposure vaccination failures from which it may be inferred that prompt vaccination of contacts (i.e., individuals exposed to smallpox) often prevented smallpox altogether, that revaccination of previously vaccinated individuals at any time during the first week of the incubation period was largely protective, and that revaccination done even as late as the second week of the incubation period attenuated disease and prevented most deaths. Primary vaccination done within 4 days of exposure was also usually protective at least from serious illness. Modern contingency planning against the release of smallpox virus during a bioterrorist attack should therefore include the capacity for prompt tracing and (re)vaccination of all contacts. Because a growing majority of the population has never before been vaccinated against smallpox and, so, may be unreachable within 4 days, anticipatory vaccination of sections of the populations of potential target countries should be considered if the bioterrorist threat intensifies.