CDC and the U.S. Food and Drug Administration rely on state and local health departments, health-care providers, and the public to report the occurrence of adverse events after vaccination to the Vaccine Adverse Event Reporting System. With such data, trends can be accurately monitored, unusual occurrences of adverse events can be detected, and the safety of vaccination intervention activities can be evaluated. On January 24, 2003, the U.S. Department of Health and Human Services (DHHS) implemented a preparedness program in which smallpox (vaccinia) vaccine was administered to federal, state, and local volunteers who might be first responders during a biologic terrorism event. As part of the DHHS Smallpox Preparedness and Response Program, CDC in consultation with experts, established surveillance case definitions for adverse events after smallpox vaccination. Adverse reactions after smallpox vaccination identified during the 1960s surveillance activities were classified on the basis of clinical description and included eczema vaccinatum; fetal vaccinia; generalized vaccinia; accidental autoinoculation, nonocular; ocular vaccinia; progressive vaccinia; erythema multiforme major; postvaccinial encephalitis or encephalomyelitis; and pyogenic infection of the vaccination site. This report provides uniform criteria used for the surveillance case definition and classification for these previously recognized adverse reactions used during the DHHS Smallpox Preparedness and Response Program. Inadvertent inoculation was changed to more precisely describe this event as inadvertent autoinoculation and contact transmission, nonocular and ocular vaccinia. Pyogenic infection also was renamed superinfection of the vaccination site or regional lymph nodes. Finally, case definitions were developed for a new cardiac adverse reaction (myo/pericarditis) and for a cardiac adverse event (dilated cardiomyopathy) and are included in this report. The smallpox vaccine surveillance case definitions presented in the report can be used in future vaccination programs to ensure uniform reporting guidelines and case classification.