The United States stopped vaccinating against smallpox in 1972 because the risks were judged to outweigh the benefits. The possibility of a terrorist attack using smallpox has led to renewed interest in a vaccination program. Smallpox vaccination carries considerable risks, which may be of greater concern today than in the late 1960s because of the increased prevalence of immunosuppression and atopy in the population. This paper reviews the clinical presentations of major adverse events after vaccination and the rates of occurrence of these events observed in the 1960s. The normal dynamics of the spread of smallpox is slow, and usually only persons who have had close personal contact with an overtly ill patient are affected. There are several preattack vaccination policy options, but immunization of medical workers, especially those who might have close contact with infected patients, is sufficient in the absence of a known threat of a bioterrorist attack or the identification of a smallpox-infected person.