The steady increase in the number of reported cases of Escherichia coli O157 infection is attributable to the proliferation of the organism, and to improved clinical and laboratory awareness. E. coli O157 is responsible for sporadic cases and outbreaks of diarrhoea, haemorrhagic colitis, haemolytic uraemic syndrome and thrombotic thrombocytopenic purpura. The pathogenesis of the organism is not fully understood, and the main virulence factors are thought to be the production of 'Shiga-like' toxin and adherence to intestinal mucosal cells. The majority of outbreaks have taken place in north America, Canada and the United Kingdom. The most important source of E. coli O157 infection is under-cooked ground beef and other bovine products. The infectious dose is low, person-to-person spread can occur, and direct transmission between calves and humans has been demonstrated. Infection has been acquired following contact with livestock at farm visitor centres. All diarrhoeal stools received in the laboratory should be examined for E. coli by culture and toxin detection methods. The presence of 'Shiga-like' toxin can be detected by demonstrating cytotoxicity, dot blotting, the polymerase chain reaction or by enzyme-linked immunosorbent assay. Serological techniques are useful in aiding and supporting a diagnosis. Contamination of meat by E. coli O157 probably occurs at slaughter or during processing, therefore, to prevent infection it is important to emphasise the dangers associated with the consumption of under-cooked beef products.