Escherichia coli O.157:H7 is a serious and common human pathogen that can cause diarrhea, hemorrhagic colitis, and the hemolytic uremic syndrome (HUS). During a massive outbreak of infection with E coli O157:H7 in January 1993 in Washington State, more than 600 people, mostly children, acquired symptomatic infection, and 37 were hospitalized with HUS at Children's Hospital and Medical Center in Seattle, and six at other hospitals in Washington. Twenty-one (57%) required dialysis. Nineteen (51%) had significant extrarenal pathology: gastrointestinal in 14 patients (38%), cardiovascular in 13 (35%), pulmonary in 9 (24%), and neurological in 6 (16%). Most patients were managed nonoperatively, but three required total abdominal colectomy and one a left colectomy. No child had perforation. Three patients died, all of whom had multisystem disease. The authors recommend (1) that all patients with bloody diarrhea undergo microbiological evaluation for E coli O157:H7 before any surgical intervention; (2) avoidance of antibiotics and antimotility agents in patients with proven or suspected infection with E coli O157:H7 until the safety and efficacy of such interventions have been established in controlled trials; (3) that patients with E coli O157:H7 infections be evaluated for microangiopathic changes consistent with HUS in the week after onset of diarrhea; (4) nasogastric suction for severe symptoms, and frequent abdominal evaluations, tests (electrolytes/amylase), and roentgenograms to exclude treatable abdominal disorders; and (5) institution of hemodialysis for oliguria/anuria, acidosis, or rising creatinine. The authors recommend surgical exploration for toxic megacolon, colonic perforation, acidosis unresponsive to dialysis, or recurrent signs of obstruction or colonic stricture.