In May 1995, the Scientific Institute of Public Health was informed of an outbreak of gastrointestinal illness in a congregational school in the Brussels area. The field investigation identified 24 cases with mild to severe gastrointestinal and general symptoms of acute bacterial enterocolitis. Campylobacter coli was detected in the stools of 5 patients. A retrospective cohort study suggested that a mixed salad (containing ham and feta cheese) was the probable source of infection, but the route of contamination remained unknown. The rapid investigation of such episodes of collective foodborne infections is essential for the implementation of adequate control measures.