Background: Myocarditis is a recognized but rare complication of smallpox immunization. It typically presents within 30 days of immunization and on initial presentation shares many characteristics with acute coronary syndrome. Electrocardiogram findings, elevated cardiac enzymes, and undifferentiated chest pain require immediate implementation of therapy directed towards an acute coronary syndrome. In an austere environment, access to advanced care may be limited. Objectives: Smallpox vaccine-mediated myocarditis may present, typically within 30 days of immunization, in such a fashion that it is impossible to distinguish from acute myocardial infarction. The purpose of this article is to alert the clinician to this problem and to provide information to assist in making a suitable diagnosis and disposition in the absence of an absolute diagnosis. Case Report: We present a case of smallpox vaccine-associated myocarditis in an American serviceman deployed in Iraq, and review the literature to determine management of these cases in an emergency setting. Conclusions: This case serves to increase awareness of the association of vaccine-mediated myocarditis in the month after immunization, and the fact that it may present similar to infarction. If the clinical probability of myocarditis is greater than infarction, this will lead the clinician to different treatment modalities.