Maricopa County Department of Public Health investigated an outbreak of E. coli O157:H7 which occurred in July 2013. A total of 94 ill persons were identified. The source of the outbreak was traced to a single location of Federico's Mexican Restaurant chain located at 13132 West Camelback Road in Litchfield Park, Arizona. Exposure dates occurred between July 18 and July 30. A case-control study of 180 persons was performed, and environmental samples were collected on multiple dates. The findings from the environmental samples and the case-control study both implicated contaminated lettuce as the probable source of the outbreak and vehicle for the E. coli O157:H7 that caused illness.