Pneumonia is the leading nosocomial infection in critically ill patients. Its exact frequency is not known because of lack of specificity of the clinical diagnosis and existing standard techniques: ventilator-associated pneumonia (VAP) complicates 7 to 44 (mean 25) per 100 admissions. According to the results of matched case-control studies, VAP is associated with a prolongation of the duration of mechanical ventilation, time in ICU and hospital stay. Estimated extra costs due to VAP average 13,000 SFr per patient (28,000 SFr per survivor). Overall mortality associated with VAP ranges from 40 to 80%, varying with the severity of the underlying illness. Mortality attributable only to the infection accounts for a third of deaths in patients with VAP.