An outbreak of Escherichia coli O157:H7 infections affected residents and staff members of a Toronto, Ontario, Canada, nursing home. The outbreak was characterized by two phases: a primary wave whose source was probably a contaminated sandwich meal and a secondary wave compatible with person-to-person transmission of infection. Older age and previous gastrectomy increased the risk of acquiring the infection. Antibiotic therapy begun after the onset of symptoms was associated with a higher case fatality rate in the more severe cases, possibly because patients with more severe disease tended to be treated with antibiotics. There were no complications or deaths among the affected members of the staff.