Although appropriate antibiotics may improve survival in patients with bacterial pneumonia, use of empirical broad-spectrum antibiotics in patients without infection is potentially harmful, facilitating colonization and superinfection with multiresistant microorganisms. Invasive diagnostic methods, including bronchoalveolar lavage and/or protected-specimen bronchial brushing, could improve identification of patients with true bacterial pneumonia and facilitate decisions whether or not to treat, and thus clinical outcome. Bronchoalveolar lavage and/or protected-specimen bronchial brushing permit collecting distal pulmonary secretions with minimal or no upper-airway contamination, either through a fiberoptic bronchoscope or blindly using an endobronchial catheter that is wedged in the tracheobronchial tree. Due to the inevitable oropharyngeal bacterial contamination that occurs in the collection of all respiratory secretion samples, quantitative culture techniques are always needed to differentiate oropharyngeal contaminants present at low concentration from higher-concentration infecting organisms. Because even a few doses of a new antimicrobial agent can negate results of microbiologic cultures, pulmonary secretions in patients suspected of having developed pneumonia should always be obtained before new antibiotics are administered. Bronchoalveolar lavage may also provide useful clues for the diagnosis of other forms of respiratory failure, such as pulmonary hemorrhage or other types of infections, especially in immunocompromised patients.