Escherichia coli O157:H7 is a well-described cause of hemorrhagic colitis in isolated cases and outbreaks. The postdiarrhea complications of this infection (thrombotic thrombocytopenic purpura and hemolytic uremic syndrome) have historically been linked to illness in children aged 5 to 10, but in an elderly, institutionalized population, E. coli O157:H7 is associated with high morbidity and mortality. This geriatric population is at high risk for developing gastrointestinal infections for a number of reasons, including age- and medication-related achlorhydria, antibiotic usage, and comorbid medical conditions. The combination of age-related risk factors with those associated with group living makes nursing facilities a high-risk environment for outbreaks of infectious diseases. E. coli O157:H7 may be more likely to cause disease outbreaks in this population because of the low inoculum required for clinical infection. Moreover, the prevalence of potential competing diagnoses, such as lower gastrointestinal bleeding from neoplastic or diverticular disease, complicates the diagnosis. Clinical presentation and laboratory studies are unpredictable and pose diagnostic challenges. This report reviews the literature on nursing home outbreaks of E. coli O157:H7 and presents an outbreak that occurred in an assisted living community in San Mateo County, California, in October 2003. The purpose of this literature review and report of an outbreak is to heighten awareness of the unique susceptibility of elderly, institutionalized patients for E. coli O157:H7 infection and its sequelae.