During January 24-March 21, smallpox vaccine was administered to 25,645 civilian health-care and public health workers in 53 jurisdictions as part of an effort to prepare the United States in the event of a terrorist attack using smallpox. Seven cases of cardiac adverse events have been reported among civilian vaccinees since the beginning of the smallpox vaccination program. In addition, 10 cases of myopericarditis have been reported among military vaccinees. This report summarizes data on the seven cases reported among civilians and provides background information on recent military vaccinees. Although a causal association between vaccination and adverse cardiac events in the civilian population is unproven, as a precautionary measure, CDC recommends that persons with physician-diagnosed cardiac disease with or without symptoms (e.g., previous myocardial infarction, angina, congestive heart failure, or cardiomyopathy) be excluded from vaccination during this smallpox preparedness program.