During a 6-mo. period in 1983, E. coli 0157:H7 was isolated from 19 (15%) of 125 patients with grossly bloody diarrhea and 1 sibling with non-bloody diarrhea in the Calgary [Alberta, Canada] area. There was no clustering of the cases geographically or in time. All but 1 had clinical manifestations typical of hemorrhagic colitis associated with E. coli 0157:H7. The illness appeared to be associated with consumption of hamburgers by 15 patients. The diarrheal illness was usually self-limited, but 3 children developed the hemolytic-uremic syndrome shortly after onset of illness. The organism was excreted in the stools very briefly in adults, although bacterial shedding continued for a longer period in children. All isolates produced verotoxin and cytotoxic activities were present in stool filtrates. The incidence of sporadic cases of hemorrhagic colitis due to E. coli 0157:H7 may be higher than has been suspected; patients with grossly bloody diarrhea should be studied promptly for E. coli 0157:H7 infection. Specific techniques for identifying this serotype must be applied to the stool cultures. Detection of free cytotoxin in stool filtrates may be an effective diagnostic procedure.