An outbreak of an upper respiratory tract illness at a secondary school, which led to the hospitalisation of 23 cases, occurred in Ireland at the beginning of September 2003. Medical and laboratory examinations, initially for suspected meningitis, were carried out on the hospitalised cases and bacterial meningitis was ruled out. One hundred and seventy-nine students and teachers were interviewed and a retrospective cohort study was conducted among the sixth year students. One hundred and seven respondents met the case definition, but no associations were found between the environmental exposures investigated and illness. As it was before the expected influenza season, initial samples were not tested for influenza, but one month later the new influenza A/Fujian/411/2002 (H3N2)-like strain was confirmed and implicated in the outbreak. It was the first reported outbreak of influenza in Europe in the 2003 'winter' season and it demonstrated the need for vigilance for early and unexpected occurrence of influenza. It also provided valuable lessons for laboratory and epidemiological investigation and management of pre-season influenza.