The study aimed to assess prevalence and risk factors for nosocomial infection (NI) in 21 hospitals of the Veneto Region (Italy).In May 2003, a one-week-period prevalence study of NI was carried out in 21 hospitals, representing 63% of all hospital beds for acute patients of the Veneto Region. Intensive care units represented 84% of all intensive care beds of the Region. Long term care, neonatal intensive care, burn, psychiatric and dermatology units were excluded.Overall, 6,352 patients were surveyed. The prevalence of NI was 7.6% (range 2.6%-17.7%), while 6.9% of patients (range 2.6%-15.5%) were affected by at least one NI. The prevalence of patients with NI in medical, surgical and intensive care areas was 6.6%, 5.0% and 25.8%, respectively. The sites most frequently affected were the following: urinary tract (28.4%), surgical site (20.3%), blood stream (19.3%), pulmonary and lower respiratory tract (17.6%). At multivariate analysis risk factors independently associated to NI were: Charlson index score >1, severity of underlying disease, exposure to antibiotics, surgical intervention, trauma at admission, presence of central venous catheter >24 h, urinary catheter, intubation, tracheostomy, and duration since admission >15 days.The study provided baseline data of NI in the Veneto Region hospitals. It showed that NI are frequent, and display a wide inter-hospital variability of rates. The highest prevalence has been reported in intensive care units. The unusual high frequency of blood stream infections and the relatively lower prevalence rate of surgical site infections highlighted the limits of prevalence studies.