Laboratory reports of a multiresistant strain of Salmonella typhimurium definitive type (DT) 104 rose in 1993; this led the Public Health Laboratory Service to investigate cases and identify possible risk factors for infection. Information derived from questionnaires, and details of previous isolations of S. typhimurium DT104 from food and animals, were used to design an unmatched case control study. Eighty-three cases whose isolations were of the same plasmid profile type (the 'epidemic strain') and 235 controls were included in the analysis. Illness was independently associated with the consumption of several food items and contact with animals, particularly ill farm animals. The number of isolations of this organism continues to rise, and control measures may include reducing infection in animals used for food, reducing the risk of contamination at all stages of the food chain, and raising awareness of measures to prevent food poisoning among food handlers and the general public.