This document updates and replaces CDC's previously published "Guideline for Prevention of Nosocomial Pneumonia" (Infect Control 1982;3:327-33, Respir Care 1983;28:221-32, and Am J Infect Control 1983;11:230-44). This revised guideline is designed to reduce the incidence of nosocomial pneumonia and is intended for use by personnel who are responsible for surveillance and control of infections in acute-care hospitals; the information may not be applicable in long-term-care facilities because of the unique characteristics of such settings. This revised guideline addresses common problems encountered by infection-control practitioners regarding the prevention and control of nosocomial pneumonia in U.S. hospitals. Sections on the prevention of bacterial pneumonia in mechanically ventilated and/or critically ill patients, care of respiratory-therapy devices, prevention of cross-contamination, and prevention of viral lower respiratory tract infections (e.g., respiratory syncytial virus [RSV] and influenza infections) have been expanded and updated. New sections on Legionnaires disease and pneumonia caused by Aspergillus sp. have been included. Lower respiratory tract infection caused by Mycobacterium tuberculosis is not addressed in this document. Part I, "An Overview of the Prevention of Nosocomial Pneumonia, 1994, provides the background information for the consensus recommendations of the Hospital Infection Control Practices Advisory Committee (HICPAC) in Part II, Recommendations for Prevention of Nosocomial Pneumonia." Pneumonia is the second most common nosocomial infection in the United States and is associated with substantial morbidity and mortality. Most patients who have nosocomial pneumonia are infants, young children, and persons > 65 years of age; persons who have severe underlying disease, immunosuppression, depressed sensorium, and/or cardiopulmonary disease and persons who have had thoracoabdominal surgery. Although patients receiving mechanically assisted ventilation do not represent a major proportion of patients who have nosocomial pneumonia, they are at highest risk for acquiring the infection. Most bacterial nosocomial pneumonias occur by aspiration of bacteria colonizing the oropharynx or upper gastrointestinal tract of the patient. Because intubation and mechanical ventilation alter first-line patient defenses, they greatly increase the risk for nosocomial bacterial pneumonia. Pneumonias caused by Legionella sp., Aspergillus sp., and influenza virus are often caused by inhalation of contaminated aerosols. RSV infection usually occurs after viral inoculation of the conjunctivae or nasal mucosa by contaminated hands. Traditional preventive measures for nosocomial pneumonia include decreasing aspiration by the patient, preventing cross-contamination or colonization via hands of personnel, appropriate disinfection or sterilization of respiratory-therapy devices, use of available vaccines to protect against particular infections, and education of hospital staff and patients. New measures being investigated involve reducing oropharyngeal and gastric colonization by pathogenic microorganisms.