An outbreak of gastroenteritis caused by Campylobacter infection was identified in May 2003 in a school in Madrid, Spain. Eighty one cases were identified in a total of 253 people studied. A retrospective cohort study showed that a custard made with ultra high temperature (UHT) milk was associated with illness (RR: 3.15; 95% CI: 1.25-7.93). The custard was probably contaminated with Campylobacter jejuni from a raw chicken prepared a day previously in the same kitchen. Our recommendations were to periodically remind the school s authorities how to act if an outbreak should be suspected, to include the monitoring of a food handler s working day in each environmental investigation in order to detect any risk behaviour; to implement microbiological analysis from the surfaces and utensils of the collective kitchens and improve the sanitary education of food handlers.