An outbreak of diarrhoea and vomiting began at a hospital for the elderly on the 25 October 1991. The symptoms and pattern of spread suggested a viral aetiology from a point source. Electron microscopy confirmed the presence of Norwalk virus. The first wave of illness affected 37 patients and 28 staff. Secondary cases occurred on all wards. Analytical studies of food consumed at three lunchtime meetings by staff and visitors to the hospital demonstrated a clear association between eating sandwiches and subsequent illness. The source of the outbreak was probably a food handler involved in the preparation of sandwiches. Control measures were implemented and the outbreak was declared over on the 11 November 1991. At this time 95 patients and 69 staff (including six visitors) had been affected. The implications for the future investigation and prevention of outbreaks of viral gastroenteritis in closed communities are discussed. An audit of the action taken during the outbreak resulted in the development of standards for the investigation and control of local outbreaks.