Staphylococcus aureus nosocomial pneumonia has become an important infection not only because of an apparently increasing incidence but also because of its high mortality rate. A total of 50 episodes of nosocomial pneumonia in critically ill patients in which etiologic diagnosis was well established were prospectively followed in a medical-surgical intensive care unit (ICU). S. aureus was isolated in a total of 13 episodes. In the univariate analysis the variables significantly associated with S. aureus nosocomial pneumonia were below 25 yr of age, coma, nonuse of corticosteroids, and antecedent trauma. A step-forward logistic regression analysis defined only coma as significantly influencing the risk of developing S. aureus nosocomial pneumonia. We suggest that antimicrobial drugs active against S. aureus must be included in the initial empirical antimicrobial regimen for treating nosocomial pneumonia in patients with coma. The identification of factors influencing the etiology and the possibility of earlier effective antimicrobial treatment may represent a further step in the control of nosocomial pneumonia in critically ill patients by improving its prognosis.