Fifty-two patients were studied prospectively to determine the etiology of postdiarrheal hemolytic uremic syndrome (HUS). Escherichia coli O157:H7 was isolated from 33 patients (63.4%). If stool obtained within 2 days of the onset of diarrhea was cultured for E. coli O157:H7, the recovery rate was 100%. This rate decreased to 91.7% and 33.3% if stool was cultured for this pathogen 3-6 or greater than or equal to 7 days, respectively, after diarrhea began. The culture-positive group was more likely to have had bloody diarrhea and fecal leukocytes and to have received transfusions than the culture-negative group but was otherwise similar in clinical characteristics. E. coli O157:H7 is the predominant pathogen associated with HUS in western Washington. Recovery of this pathogen is highly dependent on obtaining stool cultures within 6 days of onset of diarrhea.