In 1982, investigation of two outbreaks of a distinctive bloody diarrheal syndrome led to the identification of a new bacterial pathogen, Escherichia coli O157: H7 (1). In the decade since, this organism has emerged as an important public health concern on the North American and European continents, and epidemiologic, clinical, and laboratory investigations have rapidly expanded our knowledge about it and the diseases it causes. It was soon recognized that other E. coli serotypes shared a similar pathogenic potential, and the group was called the enterohemorrhagic E. coli (2). E. coli 0157: H7 is the most common and most studied member of this group. Infections caused by this organism are being recognized more frequently, which in part reflects increased interest in the organism and the availability of commercial reagents for its detection, but also a real increase in its incidence and geographic scope. The distinctive colitis it causes appears to have been uncommon until recently; outbreaks of this disease would have attracted close scrutiny whenever they