A prospective multicenter study concerning the incidence, onset time, risk factors and mortality of pneumonia was carried out by the Intensive Care Units Collaborative Group for Infection Control in Lombardy, Northern Italy. Out of 1304 patients admitted over 3 months in 16 intensive care units (ICUs), 441 met the criteria for the protocol (no previous pulmonary infection or irreversible terminal illness, ICU stay greater than 48 h). The incidence of acquired pneumonia was 21.3% (94/441), with 54.2% of cases diagnosed within 4 days of admission (early onset pneumonia). Impairment of airway reflexes on admission and more than 24 h respiratory assistance were shown as significant risk factors (RR) for early onset pneumonia (respectively RR = 12.4, with 95% confidence interval (CI) = 5.3-28.9 and RR = 3.3, with 95% CI = 1.8-5.9). A suggested pathogenetic mechanism is aspiration of oropharyngeal contents at the onset of acute illness, due to depression of protective reflexes with delayed clearance of bacterial contamination. No protection was offered by routinely applied prophylactic antibiotic therapy.