In the middle of July, 1996, a massive outbreak of hemorrhagic colitis (HC) occurred among elementary schoolchildren in Sakai city. This is the most widespread outbreak of O157 infection ever experienced to our knowledge. Lunch foods supplied in the elementary schools in Sakai were contaminated by Escherichia coli (E. coli) O157. One hundred and twenty-one cases developed hemolytic uremic syndrome (HUS) from 12,680 symptomatic patients, including putative secondary infections, and three girls died during this outbreak. Sakai City Hospital is one of the core medical facilities in this community; hence, 425 children with HC were treated at the hospital. Antibiotics were used extensively on all patients. Among them, 12 children developed HUS. All 425 children, including the patients with HUS, recovered without significant sequelae. In the present paper, the clinical experiences during this massive outbreak of E. coli O157 infection in Sakai City Hospital are described.