A confirmed outbreak of E. coli O157:H7 was associated with drinking water at a fairgrounds in Medina County, Ohio. An initial cluster of cases was revealed in August among people who attended the county fair. At that time, the environmental investigation suggested that there had been contamination in a section of the water distribution system that supplied various fair vendors, however water sampling failed to find the presence of coliform bacteria. A subsequent cluster of cases occurred among children who attended a Halloween party on the fairgrounds. The E. coli O157:H7 strains isolated were a genetic match to one another.