Escherichia coli 0157: H7 is an important and common pathogen of the human gastrointestinal tract. Acute hemor-rhagic colitis, its most frequent manifestation, can mimic other serious disorders. Because of the life-threatening disease that E. coli 0157: H7 can cause, all physicians treating patients with diarrhea should be familiar with the epidemio-logical, pathogenic, and clinical features of infection with this organism. This article first reviews the history, patho-genic properties, and nomenclature of E. coli 01 57: H7 and other toxigenic strains of E. coli and then discusses clinical aspects of infection due to these organisms.