Unpasteurized butter was the probable cause of an outbreak of E. coli O157:H7 that began at the Prospect Elementary School. A teacher made butter from unpasteurized milk and served it during a holiday party at the school. Children spread the E. coli O157:H7 to family members and to day care programs and schools. Shiga toxin, but no E. coli O157, was detected in manure from the butter maker’s milking cows.