BACKGROUND: Enterotoxigenic Escherichia coli (ETEC) has been well known pathogen as a cause of travelers' diarrhea. Nowadays, however, ETEC is also increasingly recognized as the cause of foodborne or waterborne outbreaks. We investigated an outbreak of a high school in Gwangju metropolitan city to trace the source of infection and the mode of transmission of ETEC. MATERIALS AND METHODS: All symptomatic persons were interviewed and filled out the questionnaires. We surveyed their clinical symptoms and the foods that they ate. We checked the facilities of the school, dinning room and water supply system. Microbiologic examination were carried out on above symptomatic persons and many environmental specimens. All gathered samples were examined in Gwangju city Health & Environment Research Institute. ETEC isolates were submitted to Korea Center for Disease Control and Prevention (KCDC) for Pulsed-field gel electrophoresis (PFGE). We also requested water quality analysis of water samples to Waterworks Research Institute. RESULTS: The 39 students and one sibling who visited and ate school meal showed symptoms. All staffs and cooks showed no symptom. Among 51 stool specimens, ETEC was isolated from 19 (18: students, 1:cook). ETEC was also isolated from specimens from a purifier and a water tap of the dinning room and water tank (underground water). In PFGE test, isolates from 18 persons showed identical pattern, but the PFGE patterns from water were different. Isolates of water samples showed different PFGE patterns even within the same sample. Investigation of the water distribution system revealed that contaminated underground water had been supplied to the dinning room and students of the school were exposed to it. CONCLUSION: This is the outbreak of ETEC infection occurred in a high school. Besides students, ETEC was also isolated from water samples. Considering some epidemiologic features, we suspect the mode of transmission may be waterborne despite the inconsistent PFGE results.