CDC and the World Health Qrganization (WHO) are continuing to investigate the multicountry outbreak of unexplained atypical pneumonia referred to as severe acute respiratory syndrome (SARS). Pending development of confirmatory laboratory testing capacity, CDC's interim suspected SARS case definition is based on clinical criteria and epidemiologic linkage to other SARS cases or areas with community transmission of SARS. This case definition will be updated periodically as new information becomes available. Epidemiologic and laboratory investigations of SARS are ongoing. As of April 2, 2003, a total of 2,223 suspected and/or probable SARS cases have been reported to WHO from 16 countries, including the United States. The reported SARS cases include 78 deaths (case-fatality proportion: 3.5%). This report summarizes SARS cases among U.S. residents and surveillance and prevention activities in the United States.