Since 1982, enterohaemorrhagic Escherichia coli (EHEC) have been identified as a cause of diarrhoea and haemorrhagic colitis. The most serious complication of the infection is the haemolytic-uraemic syndrome (HUS) that develops in 5 to 10% of children with diarrhoea. Shiga toxins (Stx) are the most important presently known virulence factors of EHEC. After reaching the bloodstream, the toxins cause damage of endothelial cells but also of tubular cells in the kidneys which may result in renal failure. In EHEC O157 isolates from patients, we were able to identify seven different combinations of stx genes that occurred with different frequency. The genes encoding Stx are located in the genomes of prophages that are integrated in EHEC chromosomes. In addition, EHEC O157 strains possess a chromosomally located pathogenicity island (pais) termed LEE that contains numerous pathogenicity genes including the eae gene encoding intimin. Moreover, EHEC O157 strains possess a 93-kb plasmid harbouring genes encoding the EHEC-hemolysin (EHEC-HlyA), a serin protease (EspP) that cleaves factor V and a protein called ToxB that shows homology with the toxin B of Clostridium difficile. The EHEC O157 strains exist in two variants, namely non-sorbitol-fermenting (NSF) O157:H7 and sorbitol-fermenting (SF) O157:H- strains that are evolutionary older. Our results obtained up to now demonstrate marked differences in epidemiology of the infection caused by the respective EHEC O157 variants. EHEC O157:H7 strains occur worldwide, whereas SF strains have been hitherto found only in Germany and recently also in the Czech Republic. While the EHEC O157:H7 strains occur mainly during warm months, the SF strains are more frequent during the cold season of the year. In addition, differences exist with regard to the resistance to heavy metals, the plasmid structure, and the reservoir. We postulate that SF O157:H- strains occur only in the human intestine, whereas NSF O157:H7 strains have become adapted to other hosts, such as cattle, promoting a more rapid spread of the strains.