Nosocomial infections are a major challenge for modern medicine and contribute to increased resource use in health care systems. The first hospital-wide prevalence survey of nosocomial infections was conducted at the University of Geneva Hospitals in 1994. At the time of the study, 16.9% of admitted patients had nosocomial infections (168/994). Leading infection sites were: urinary tract (30%), respiratory tract (17%), surgical wounds (12%) and bloodstream (9.6%). Rates of infection varied between hospital wards: intensive care (21%), surgery (19%), rehabilitation (18%), internal medicine (13%). However, the distribution of nosocomial infections varied according to surveillance and attribution rules. Optimal detection of nosocomial infections requires ward surveillance, including revision of microbiology, nursing (Kardex) and medical records; the combination of fever above 38 degrees C, prescription of antimicrobial agent(s), and positive microbiological records suggested the diagnosis of nosocomial infections in 95% of situations. Priorities for infection control were derived from these results.