During 1979-82 hospital discharge rates after emergency admission for acute appendicitis were higher in Eire (17.4 per 10,000 population) than in England (10.3), Scotland (11.1), or Wales (12.3). Comparison of food consumption between the four countries, and between the health board areas of Eire and regions of Scotland, shows that appendicitis rates are highest in communities that consume more potatoes, sugar, and cereals, and less non-potato vegetables and fruit. The rates are also related to mortality from enteric and respiratory infections, to general practice consultations for infective disease, and to postneonatal mortality. This is evidence in favour of the hygiene hypothesis for the aetiology of appendicitis.