Although epidemiologic investigations of hospital-acquired pneumonia have certain intrinsic limitations because of the heterogeneity of the study populations, the difficulties in making a clinical diagnosis of nosocomial pneumonia, and the need for better microbiologic assays, recent studies have provided new and important data concerning the role of Staphylococcus aureus in this disease. This pathogen has now been identified as the most frequent cause of nosocomial pneumonia in hospitals in both Europe and the United States among patients in general hospital units as well as in the intensive care unit (ICU). Patients who have been treated with mechanical ventilation are at especially high risk for S. aureus pneumonia. The incidence of nosocomial pneumonia related to methicillin-resistant S. aureus (MRSA) has increased in recent years in many countries, especially among patients in the ICU. Because hospitalized patients with suspected nosocomial pneumonia often have many risk factors for MRSA infection, it seems advisable to include coverage of MRSA in the initial therapeutic regimen for these patients until MRSA infection is excluded.