An outbreak of E. coli O157:H7 infections occurred among persons who had visited a swimming beach within an Illinois state park. A matched, case-control, analysis study suggested that swimming at the park was the only risk factor for illness. Risk of illness was also associated with taking lake water into the mouth and with swallowing lake water. Fecal coliform levels of lake water, measured on July 10 after the water had been closed for swimming for four days, showed high fecal coliforms. This suggested that there could have been contamination of the water by nonhuman sources.