In December 2002, the federal government began a program to immunize approximately 500000 civilian public health and health care workers with smallpox (vaccinia) vaccine as a part of our pre-event defense against bioterrorism. First responders will likely follow, and the general US population might be offered vaccination in the next 1 to 2 years. Recent reports that suggest the possible association of the vaccine to adverse cardiac events (including deaths), liability concerns for hospitals, and the availability of compensation for workers with vaccine complications have significantly reduced voluntary participation. Vaccinees might experience robust primary takes or serious adverse events, including viral or even bacterial cellulitides, encephalitis, progressive skin destruction, and other life-threatening complications. With the increasing prevalence of immune suppression from both diseases and immunosuppressive medications, complications might be seen in higher frequency than previously reported. Emergency medicine providers and staff must become familiar with clinical presentations and management of vaccine complications. In addition, policies and procedures must be developed to prevent unimmunized providers from inadvertently contacting the active vaccination sites of their patients and, if the providers themselves have active vaccination sites, to protect their patients and their own families.