We identified retrospectively 19 critically ill, intubated patients in whom tracheobronchial secretions had been cultured before antimicrobials were administered. Each of these patients had bacterial nosocomial pneumonia, as diagnosed by a positive bacterial culture from blood, pleural fluid, percutaneously aspirated lung material, special endobronchial brushings, and/or postmortem lung specimens. In 11 (58%) patients, the predominant organism in the tracheobronchial secretions was the same one identified in the diagnostic culture. However, in eight (42%) patients, the infecting bacteria were either not predominant or did not grow in the culture of respiratory secretions. We conclude that standard microbiologic studies of central tracheobronchial secretions are not always reliable for establishing the etiology of a bacterial nosocomial pneumonia in this type of patient.