Verotoxigenic Escherichia coli are a recently described class of diarrheagenic E. coli . The most prominent member of this group, serotype O157:H7, is a well-recognized cause of hemorrhagic colitis and hemolytic uremic syndrome. This serious human pathogen has caused numerous outbreaks in the developed world and has contaminated such widely disparate foods as ground beef, apple cider, and lettuce. Serotypes other than O157:H7 have also been found to cause sporadic enteric disease and several outbreaks have been recently described. The non-O157 SLTEC are more frequently present in food animals and foods of animal origin than serotype O157:H7. Particular non-O157 serotypes (such as O26) have a definite association with HUS. Surveillance data from several regions suggests that there may be important differences in the distribution of serotypes causing HUS in different geographic areas. While more than 100 serotypes of E. coli have been identified as possessing one or more SLT genes, far fewer than that number have been convincingly associated with human illness. Current research needs to determine those additional virulence traits which confer pathogenicity on organisms possessing the SLT gene. Equally important will be to ascertain the relative contribution of different serotypes to human disease in order to develop sound, scientifically based, control strategies.