The results of a study on the incidence of nosocomial infections in a 1800 bed University hospital are reported. The study, carried out over a 9 months period, included: continuous microbiological surveillance, and a clinical and epidemiological survey. On the basis of the microbiological data collected and analyzed by a computer data system, developed and employed for the control of nosocomial infections, a weekly bed-to-bed survey was carried out by the staff of the Institute of Infectious Diseases. Among 2777 suspected nosocomial infections, as revealed by microbiological monitoring, 701 were confirmed after the bed-to-bed survey. The nosocomial infection rate was 6.75 per 100 discharges. It was higher in the surgical than in the medical wards (7.3 and 6 per 100 discharges, respectively). Nosocomial urinary tract infections were the most frequent (74.2%). The urinary infection rate was higher in the surgical than in the medical wards (5.3 and 4.6 per 100 discharges, respectively). Escherichia coli (19.4%), Pseudomonas aeruginosa (19.3%), Proteus spp. (18.4%) were the pathogens most frequently associated with nosocomial infections. They were followed by Klebsiella pneumoniae (7.8%) and Staphylococcus aureus (6.5%) in frequency. Among the risk factors, involved in nosocomial infections, the importance of catheterization was confirmed: among our patients with nosocomial urinary tract infections, 73.4% and 79.5%--in the medical and surgical wards, respectively--underwent urological instrumentation, mainly catheterization. An analogous and more detailed study is now in progress and will be extended in the next years.