An outbreak of E. coli O157:H7 occurred among attendees of two picnics held on September 5 and 6 at a Connecticut country club. The hamburger patties were heavily contaminated with E. coli O157:H7. The E. coli O157:H7 isolates from the case-patients were a genetic match to the strain isolated in the beef. Illness risk was increased for those who were served medium and rare hamburgers. According to the chef, some of the hamburgers were still frozen at the time of cooking and may not have been cooked all the way through. The beef had come from via a New York distributor from one of two large, mid-western, beef packing companies. Six large slaughter houses supplied the beef packing companies.