On March 7, 2007, the Chicago Department of Public Health and the University of Chicago Pediatric Infectious Disease Service and Infection Control Program notified CDC of a child with presumed eczema vaccinatum (EV), a life-threatening complication of vaccinia virus infection. This is the first reported EV case in the United States since 1988. This report summarizes the epidemiologic and environmental investigations conducted by local, state, and federal public health authorities in Illinois and Indiana to determine the source of exposure and to identify and monitor other persons at risk for vaccinia virus infection. This case highlights the need for clinicians to maintain a high index of suspicion when evaluating recently vaccinated patients and their family members with vesiculopustular rash.