CDC, several states, and the U.S. Food and Drug Administration are investigating a multistate outbreak of Shiga toxin-producing E. coli O157:H7 infections (STEC O157:H7) in 13 states. Seventeen illnesses have been reported from California (3), Connecticut (2), Illinois (1), Indiana (1), Michigan (1), Nebraska (1), New Hampshire (2), New York (1), Ohio (1), Pennsylvania (1), Virginia (1), Vermont (1) and Washington (1). Illnesses started on dates from November 15 through December 8, 2017. The Public Health Agency of Canada also is investigating an outbreak of STEC O157:H7 infections in several provinces. There are now a total of 41 cases of E. coli O157 under investigation. There was one reported death. There appears to be an ongoing risk of E. coli infections associated with the consumption of romaine lettuce in Canada’s eastern provinces. CDC is performing whole genome sequencing on samples of bacteria making people sick in the United States to give us information about whether these illnesses are related to the illnesses in Canada. Preliminary results show that the type of E. coli making people sick in both countries is closely related genetically, meaning the ill people are more likely to share a common source of infection. The Public Health Agency of Canada has identified romaine lettuce as the source of the outbreak in Canada. In the United States, state and local public health officials are interviewing sick people to determine what they ate in the week before their illness started. CDC is still collecting information to determine whether there is a food item in common among sick people, including leafy greens and romaine. Because we have not identified a source of the infections, CDC is unable to recommend whether U.S. residents should avoid a particular food. This investigation is ongoing, and more information will be released as it becomes available.