We studied the efficacy and safety of bronchoscopy with bronchoalveolar lavage (BAL) in mechanically ventilated patients. Seventy-seven patients, 60 of whom underwent BAL, were analyzed. Of the patients undergoing BAL, 30 had clinical pneumonia, 24 had a diagnosis other than pneumonia by clinical criteria or autopsy, and six could not be classified but clinically improved without changing their antibiotic therapy. Of the 30 pneumonia patients, 18 had bacterial cultures felt to be diagnostic of bacterial pneumonia: two cases of Legionella pneumophila, and 16 cases with one or more organisms recovered at greater than 10(4) cfu/ml of BAL fluid. No patient without the clinical diagnosis of pneumonia had a positive bacterial culture greater than 10(4) cfu/ml of BAL fluid (chi-square = 18.2, p less than .001). Of the patients classified with pneumonia, Pneumocystis carinii was found in six and cytologic evidence of viral infection in three patients. Of the 30 patients undergoing BAL with pneumonia, 27 had one or more pathogens identified in the lavage specimen. Although no patient died as a result of lavage, significant hypoxemia was encountered in some patients undergoing lavage. In 35 patients with the same FIO2 before and after bronchoscopy, the median change in PO2 was -8.0 torr (range -63.0 to +29.0). We found that bacterial cultures of BAL fluid appeared useful in defining the presence and etiology of pneumonia.