Variola virus, the cause of smallpox disease, has been deemed a possible bioterrorism agent. Since November 2001, federal, state, and local public health partners implemented activities to prepare for a possible smallpox outbreak. The Centers for Disease Control and Prevention (CDC) produced and delivered training and educational materials for smallpox preparedness in many formats, developed detailed smallpox vaccine information statements about vaccine contraindications and vaccination site care, and established mechanisms to monitor and respond to adverse events after smallpox vaccination. The last included enhancements to the Vaccine Adverse Event Reporting System, a pregnancy registry for inadvertently vaccinated pregnant women, and a Clinician Telephone Information Line to collect reports about adverse events. The civilian responder vaccination program was conducted with rigorous safety procedures, and few historically recognized adverse events were observed. However, myocarditis and/or pericarditis was newly recognized as an adverse event caused by the New York City Board of Health vaccinia vaccine strain. This smallpox preparedness program put into place a number of measures to advance the United States' readiness for a smallpox outbreak that have assisted in preparedness for other threats.