Quantitative bacterial cultures of protected specimen brush (PSB) and bronchoalveolar lavage (BAL) specimens were used to assess the presence of distal bronchial microflora in mechanically ventilated patients who were not suspected of having pneumonia. A total of 54 specimens from 32 ICU patients were obtained. In 10 patients, only PSB specimens were recovered, whereas in 22 patients both PSB and BAL specimens were obtained. Cultures demonstrated no growth in 10 (45%) of the 22 samples recovered by BAL and in 21 (66%) of the 32 samples recovered by the PSB method. Twenty-three specimens yielded microorganisms, but in only six patients was a significant growth in PSB and/or BAL cultures (> or = 10(3) and > or = 10(5) cfu/ml, respectively) detected. Four of these patients developed pneumonia on subsequent follow-up. In mechanically ventilated patients, the diagnostic thresholds of bacterial growth in PSB and BAL cultures may be misleading. A significant relationship was found between the ability of PSB to recover microorganisms and the duration of mechanical ventilation (p < 0.05). These findings may be a limitation present in all patients receiving prolonged mechanical ventilation.