Several elementary school principals in Haute-Garonne and Tarn simultaneously notified health authorities of mild digestive problems among students. A retrospective cohort study, using self-administered individual questionnaires completed by approximately 3,000 students and teachers who had participated in two meals at 36 affected schools, was initiated to confirm the existence of a foodborne illness outbreak and to identify its source. The analysis was conducted both overall and separately by meal. Concurrently, a veterinary investigation was conducted. Among the 2,266 respondents, the overall incidence rate was 50%. For those who consumed only one of the meals, the respective incidence rates were 54% and 29%, and the median incubation periods were 32 hours and 6 hours. These results suggested two hypotheses: a foodborne illness of viral origin, or two consecutive foodborne illnesses with two distinct causes, one viral and the other due to staphylococcal enterotoxin, consistent with the results of the food and microbiological investigations. This large-scale foodborne illness outbreak illustrates the main factors contributing to the occurrence of foodborne illnesses (multiple failures in meal preparation) and highlights the importance of rapidly integrating epidemiological, veterinary, and microbiological investigations with initial management measures (removal of sick staff) during the response to an alert, in order to avoid major consequences in institutional food service.(R.A.)