The purpose of this prospective study was to compare the results of bacteriologic assessment of the lower respiratory tract in mechanically ventilated patients by two sampling techniques: fiberoptic protected brush biopsy or blind bronchial aspiration of secretions. Thirty-three mechanically ventilated patients with acute bronchopneumonia were studied. Paired samples were collected by both techniques 47 times. Identical results were obtained in 93% of the cases. In 28 pairs, one or several identical organisms were cultured. In 16 pairs, no bacteria were isolated; in the three remaining pairs, results were contradictory. In five patients with concomitant positive blood cultures, the same organisms were cultured from fiberoptic protected brush biopsy, blind bronchial sampling, and blood cultures. These findings support the fact that blind bronchial sampling can be recommended for bacteriologic assessment of the lower respiratory tract in mechanically ventilated patients with diffuse nosocomial bronchopneumonia.