An outbreak of E. coli O157:H7 infections occurred among people who attended the Lorain County Fair in Ohio. Visiting a specific show barn was associated with illness. Among barn visitors, illness was independently associated with dancing, eating/drinking, or handling saw dust in the barn. The outbreak strain of E. coli O157:H7 was recovered from barn sawdust, bleachers, handrails, walls, and rafters from specimens collected six weeks after the outbreak. Sawdust specimens collected 42 weeks after the fair also grew the same E coli O157 strain.