Nosocomial pneumonia remains an important problem in patients undergoing mechanical ventilation, being associated with high mortality and morbidity and considerable expenditure. In the past respiratory equipment has been implicated in the development of nosocomial pneumonia and strict recommendations for cleaning and maintenance have been practiced. It is now known that the circuit and other equipment rapidly become contaminated with microorganisms originating from the patient's upper airway flora. These organisms access the circuit through suctioning and coughing, and may contaminate distant sites by traveling in association with aerosols or condensate. Current evidence suggests that circuit contamination usually is a result rather than a cause of airway colonization and does not have an important role in the pathogenesis of nosocomial pneumonia. Provided that reasonable infection control measures are taken, circuit contamination does not pose a risk to the ventilated patient. Although bacterial filters placed in the circuit effectively prevent circuit contamination they do not significantly reduce the incidence of nosocomial pneumonia in patients receiving mechanical ventilation.