An outbreak of diarrhea was associated with visits to a dairy farm in Montgomery county, Pennsylvania. Over 75,000 persons had visited the farm during the outbreak period. Case-patients were more likely to have been in contact with cattle. The E. coli O157:H7 found in the stool of case-patients matched the E. coli O157:H7 that had been isolated from cows and environmental surfaces on the farm. Handwashing facilities were poorly stocked with soap and towels, were out of a child's reach, or were poorly supervised.