Escherichia coli O157:H7 is a recently recognized enteric pathogen that causes acute hemorrhagic colitis. Although the infection is usually self-limited, it may be complicated by hemolytic uremic syndrome and thrombotic thrombocytopenic purpura. At our institution, stool specimens are now routinely cultured for this organism. To determine the prevalence of E. coli O157:H7-associated diarrhea in our patient population, we surveyed all submitted stool cultures for 6 months for this organism. Specimens were screened for non-sorbitol fermenting E. coli and confirmed by slide-agglutination and immobilization testing. Of 2,164 specimens, 10 yielded E. coli O157:H7. It was the fourth most common bacterial stool pathogen found. Bloody diarrhea and abdominal pain were the most common symptoms of the infected patients. E. coli O157:H7 causes sporadic infections in our patient population and should be considered in the differential diagnosis of acute hemorrhagic colitis.