Nosocomial pneumonias have various etiologies and their development depends mainly on the underlying condition of the patients. Intubated patients are prone to development of bacterial pneumonia from the oropharyngeal or gastric flora. Prevention relies on reducing exogenous as well as endogenous colonization of the bronchotracheal tree: avoidance of cross-contamination, maintenance of a physiological gastric pH and, possibly, selective digestive decontamination. Neutropenic patients may develop invasive aspergillus infection. Prevention depends on appropriate air filtration. Patients with cellular immunodeficiency are susceptible to various agents. Prevention of legionella depends on control of the water and ventilation systems. The prevention of cytomegalovirus infection includes the screening of blood products for certain patients and, in some cases, the administration of hyperimmune gammaglobulins and possibly ganciclovir. Even though Pneumocystis carinii pneumonia is thought to be due to reactivation, recent evidence suggests that transmission may occur between patients and therefore appropriate respiratory isolation is advisable. Finally, nosocomial tuberculosis is an increasing problem in which control depends on early diagnosis and treatment of patients as well as on appropriate air exchange in particular rooms of the hospital. In conclusion, the prevention of nosocomial pneumonia includes numerous measures which largely depend on the type of microorganisms.