Three cases of hemolytic uremic syndrome (HUS) that occurred over a 15-day period at a daycare center were reported to the French Institute for Public Health Surveillance (InVS) on November 5, 2012, suggesting a cluster of infections caused by enterohemorrhagic E. coli (EHEC). An exploratory investigation, a retrospective cohort study involving 89 out of 94 children (95%) and 28 out of 36 staff members (78%), and prospective surveillance were conducted to identify a common exposure and determine the extent of the outbreak. Screening was performed among the family contacts of confirmed cases and among the children and staff. EHEC was detected by PCR (Polymerase Chain Reaction) in stool samples; positive samples were cultured for strain characterization. A confirmed case was defined as a child or staff member at the daycare center who, between October 10, 2012, and January 17, 2013, presented with signs of gastroenteritis and/or HUS with isolation of EHEC O111; a probable case was defined as a child or staff member who presented with bloody stools and/or HUS and a positive PCR result; a possible case was defined as a child who had diarrhea with at least 3 loose stools in 24 hours and consulted a doctor. Eighteen cases (6 confirmed, 1 probable, and 11 possible) were identified among the children. The epidemic curve suggested person-to-person transmission. The attack rate was 20.2%, or 6.7% when considering only confirmed cases. Attendance in Group A was significantly associated with the disease (RR=3.1, 95% CI [1.3–7.1]). Among the household contacts of confirmed cases, 17% (3/18) were asymptomatic carriers. Screening identified 4% (3/80) of children as asymptomatic carriers. In response to the issues raised by the management of the outbreak, the High Council for Public Health (HCSP) established an expert group in July 2013 to develop recommendations and prevent the occurrence of secondary EHEC cases in group settings. (R.A.)