E. coli O157:H7, the predominant serotype of EHEC, is a cause of both outbreaks and sporadic cases of hemorrhagic colitis. In sporadic cases, and especially in outbreaks, there is an association with the consumption of improperly cooked ground beef. Both young children and geriatric patients have an increased attack rate for EHEC infection as well as an increased incidence of the two sequelae of intestinal infection with EHEC, hemolyticuremic syndrome, and thrombotic thrombocytopenic purpura. The hallmark of hemorrhagic colitis due to EHEC is the development of bloody diarrhea several days after the onset of nonbloody diarrhea and abdominal pain. Fever is usually absent or low-grade. The pathogenesis of EHEC infection is probably related to at least two bacterial virulence factors: adherence of bacteria to intestinal mucosa and production of one or more cytotoxins. These cytotoxins are closely related to Shiga toxin and therefore are often referred to as Shiga-like toxins. Treatment for hemorrhagic colitis is supportive care; most illnesses are self-limited. At present, there is no evidence that antimicrobial therapy shortens the course of illness or prevents the development of sequelae.