Nosocomial respiratory tract infections are major causes of excessive morbidity and mortality in US hospitals, affecting an estimated five to ten of every 1,000 patients. Patients with serious underlying diseases have an especially high risk of acquiring these infections, and that risk is magnified by exposure to respiratory therapy. Until recently, contaminated respiratory care devices were a major cause of infection, but procedures for the management of these devices have decreased their role substantially. Now, aspiration of oropharyngeal flora appears to be responsible for most cases of bacterial respiratory infections. Therefore, techniques to alter the flora of the oropharynx and to diminish the risk of aspiration are important priorities for infection control. Exposure to intensive care units (ICUs) is also a major risk factor for nosocomial pulmonary infection, and person-to-person spread of microorganisms within ICUs seems to be responsible for some of these infections. Increasing numbers of pulmonary infections are being caused by newly identified or previously uncommon pathogens, and this shift requires that traditional control techniques be reevaluated to assure that they remain relevant. Few new techniques have been developed recently to alter host susceptibility, prevent acquisition of virulent pathogens, or eradicate infectious organisms from their sources, but effective infection control programs in hospitals appear to reduce the incidence of nosocomial pulmonary infection, and implementation of these programs deserves serious consideration in all hospitals.