An outbreak of E. coli O157:H7 occurred at an elementary school in Wisconsin. Illness was associated with eating brownies that were served in the school's self-service food bar on October 18. No brownies remained for testing. Stool testing of school lunch employees failed to find evidence of colonization with E. coli O157:H7. One student had been ill with E. coli O157:H7 eight days before the other illnesses began. The student selected items from the food bar while ill. The E. coli O157:H7 isolated from this student was a match to the isolates collected from the others. It seemed likely that a student had transmitted E. coli O157:H7 through the food bar.