In September, 2009, a cluster of patients who had been infected with an indistinguishable strain of E. coli O157:H7 was identified. Initially case-patients were identified in Colorado, Utah, and New York State. Additional case-patients were identified subsequently in South Dakota, Wisconsin, and North Carolina. The Colorado case-patients had all eaten at the same Chipotle Mexican Grill in Boulder, Colorado, on September 4, 2009. In Utah, all case-patients had eaten at the Cafe Rio Restaurant located in Salt Lake City, Utah, between August 31 and September 4, 2009. The New York State case patient had eaten at a Chipotle Mexican Grill. A case control study involving Utah and Colorado case-patients was conducted; it showed that eating romaine, or iceberg, lettuce was associated with risk of illness. The New York State case-patient had eaten romaine lettuce at the Chipotle Mexican Grill. A traceback of the romaine lettuce led to a common harvester/shipper, Church Brothers, LLC, located in Salinas, California. No lettuce remained for testing and environmental samples collected at Church Brothers, LLC, did not show the presence of E. coli O157:H7. Investigation of the cases in South Dakota, Wisconsin, and North Carolina did not provide useful information for the trace-back investigation. Lettuce was the most likely vehicle for this outbreak because of the common lettuce source for the cases in Utah, Colorado, and New York State. These cases represented 16 out of the 19 confirmed cases. Although Cotija cheese, pinto beans, and pico de gallo consumption were also associated with illness, it was likely that these results reflected confounding as lettuce is usually served with these ingredients in Mexican style restaurants. Cotija cheese was not used in the Colorado and New York Chipotle Restaurants. A common source of cilantro, the most suspect ingredient in pico de gallo, was not identified for Cafe Rio or for the Chipotle Mexican Grill Restaurants.