This paper describes the methods used to perform a very large multicentred prevalence survey of infection in hospitals. Infection control teams were trained centrally to use a standardized questionnaire and agreed definitions to collect prevalence data on a portable computer. The study was coordinated from a single centre and the analysis performed by the statistics department at Central Public Health Laboratory, Colindale, London. The survey included 157 centres throughout England and Wales, Scotland and all Ireland. The survey was carried out as a joint venture by members of The Hospital Infection Society, The Public Health Laboratory Service and the Infection Control Nurses' Association of the British Isles and was organized by a Steering Committee.