Mass sociogenic illness (MSI) is the occurrence of a group of nonspecific physical symptoms for which no organic cause can be determined and that is transmitted among members of a group by "line of sight." On April 22, 1994, the Snohomish (Washington) Health District (SHD) was notified of an outbreak of unexplained illness characterized by abrupt onset of nausea and headache among students at a middle school. This report summarizes the investigation of this outbreak by SHD, which determined that MSI was the most likely cause of the outbreak.