The reliability of a bronchoscopic protected catheter brush (BPCB) in the diagnosis of lower respiratory tract infection was studied in 17 intubated and ventilated patients, including seven patients free from such infection (group 1) and ten patients with suspected infection (group 2). A first sample was obtained in the lower trachea by aspiration through the fiberoptic bronchoscope and a second in a distal bronchus by the BPCB procedure. In group 1, all BPCB cultures were sterile, although lower tracheal cultures yielded two or more bacterial species, showing that uncontaminated specimens can be obtained by the BPCB procedure. In three patients of group 2, BPCB cultures remained sterile as a nonbacterial pulmonary disease was certified by open lung biopsy. In seven patients from group 2, BPCB cultures yielded all of the organisms isolated simultaneously by reference methods (ie, cultures of blood or pleural fluid, serologic tests, and open lung biopsy). In two of these patients, contamination of the BPCB specimens was ascertained by the reference method bacterial results. In this study the BPCB procedure was able to obtain uncontaminated specimens in intubated and ventilated patients and was mainly accurate in identifying the bacterial etiologic agents of lower respiratory tract infections.