At least 15 states reported 25 laboratory confirmed cases of E. coli O157:H7 that appeared to be linked to consumption of leafy greens. Illnesses started on dates from November 05, 2017 through December 12, 2017. Among the 18 ill people for whom the CDC had information, 9 were hospitalized. Two developed hemolytic uremic syndrome. One person from California died. Of the 13 people interviewed, all 13 reported eating leafy greens in the week before illness onset. Five of 9 ill people specifically reported eating romaine lettuce. Ill people reported eating different types and brands of romaine lettuce. As of January 10, 2018 no common supplier, distributor, or retailer of leafy greens had been identified as a possible source of the outbreak.

Whole genome sequencing showed that the E. coli O157:H7 strain from ill people in the United States was closely related genetically to the E. coli O157:H7 strain isolated from ill people in Canada. The outbreak in Canada was first reported on December 11, 2017. The Public Health Agency of Canada reported the outbreak in Canada over on January 10, 2018. Forty two cases were reported in five eastern provinces: Ontario, Quebec, New Brunswick, Nova Scotia and Newfoundland and Labrador. Seventeen individuals in Canada were hospitalized. One person died. Illness onsets ranged from November 16 to December 12, 2017. Canadian investigators determined that consumption of romaine lettuce was the source of the outbreak in Canada.

The combined total number of outbreak associated cases in both the US and Canada was 67. Twenty six people were hospitalized. Two people died.