In November 1982, 31 (8.8%) of 353 residents at a senior living facility in Ottawa, Ontario, Canada, became ill with gastrointestinal symptoms. Cases occurred over an 18-day period. None of the usual enteric pathogens (Salmonella, Shigella, Campylobacter, Yersinia, or Amoeba) were found in stool specimens obtained from the 31 affected residents. Escherichia coli O157:H7 was isolated from the stools of 17 patients, with the isolation rate higher for those with watery, bloody diarrhea (78%) than for those with loose stools only (38%). In studies of serial stool specimens, E. coli O157:H7 was usually isolated from the time diarrhea began until formed stools returned, up to 11 days after onset. Food prepared in the facility's kitchen was a possible source of the outbreak. A single lot of hamburger had been served repeatedly between November 6 and 20 and was considered the most likely vehicle. Two samples of frozen meat from this lot were cultured, but E. coli O157:H7 was not recovered.