In late November 1999, the Institute for Public Health Surveillance was notified by the National Reference Center for Salmonella and Shigella of an increase in human isolates of Salmonella Typhimurium in Department 04. Several cases involved patients in hospitals and nursing homes. An epidemiological investigation was initiated to identify the source of the outbreak and propose appropriate control measures. A case was defined as a person with a laboratory-confirmed isolation of S. Typhimurium in Department 04 since September 15, 1999. Cases were interviewed about the circumstances surrounding the onset of their salmonellosis and their food consumption in the 3 days preceding the onset of symptoms. For hospitalized cases, the medical history of the episode and food consumption were reconstructed using medical records and menus. A "cross-case" investigation was conducted for hospitalized cases to test the hypothesis generated by the exploratory food survey. Human and food strains were compared using lysotyping and DNA macrorestriction at the National Reference Center for Enteric Molecular Typing. A total of 35 cases of S. Typhimurium infection occurred between September 15, 1999, and January 7, 2000: 29 cases among patients in 6 hospitals and nursing homes, and 6 cases among children attending school cafeterias in the department. The pattern of the epidemic curve suggested a common and prolonged source of contamination. Three patients with severe underlying medical conditions died from complications related to their salmonellosis. Consumption of ground beef was significantly associated with the onset of the disease [OR=5, p<0.01]. The veterinary investigation identified two batches of frozen ground beef, positive for S. Typhimurium, from a single producer common to all these establishments. The human and foodborne strains exhibited the same DNA macrorestriction profile. A recall of the two contaminated batches was carried out. This outbreak is the third salmonellosis outbreak detected since 1990 to occur in hospitals following the consumption of frozen ground beef patties and should draw attention to the food safety risk associated with the use of high-risk foods in these settings. These incidents underscore the importance of complying with regulations governing institutional food service, the need to remind the public of the risks associated with consuming undercooked meat, and the limitations of microbiological criteria applied to high-risk foods consumed by vulnerable individuals. (R.A.)