The first case of severe acute respiratory syndrome (SARS) in France was diagnosed in March 2003. We conducted a serological survey to assess whether or not asymptomatic persons who had been in contact with this patient during his infectious stage had been infected. They were interviewed and asked to provide a blood sample for SARS coronavirus immunoglobulin G antibody testing. Despite the likely high infectivity of the SARS patient, no asymptomatic SARS infection was found in any of the 37 contacts included. These findings support a SARS case definition that is essentially based on clinical and epidemiological assessment, should SARS re-emerge. (R.A.)