The prevalence of hospital-acquired infection was measured in 47 hospitals in 14 countries in four continents. The aim was to establish the evidence that hospital infection is a common and serious problem throughout the world. Using a standard protocol, 28,861 patients were observed by local teams of doctors and nurses in their own hospitals. The prevalence rates in individual hospitals varied from 3% to 21% (median 8.4%). The highest rates were seen on intensive care (13.3%), surgical (13.1%) and orthopaedic wards (11.2%). Children under the age of 1 year (infection prevalence 13.5%) and adults over 64 years (prevalence 12.0%) suffered more infection than others. In children the commonest infections were of the lower respiratory tract, of the skin and gastroenteritis. In the elderly, urinary-tract infections predominated. The prevalence of postoperative wound infection in individual hospitals ranged from 5.2% to 34.4%, with even greater variation when the wounds were analysed as clean, clean-contaminated and contaminated. The micro-organisms isolated from infected patients were similar to previous surveys: Escherichia coli and Staphylococcus aureus each caused a sixth of the infections with positive microbiological results. When examined, 30% of patients were on antimicrobial drugs. Penicillin, ampicillin/amoxycillin and gentamicin were the commonest antibiotics used.