We prospectively investigated the diagnostic value of semiquantitative (semiQC) and quantitative (QC) cultures of endotracheal aspirate (ETA) compared with QC of bronchoalveolar lavage (BAL) fluids in 18 mechanically ventilated trauma patients with clinical signs of pneumonia. The general agreement between QC of ETA and BAL was 89% when conventional cutoffs for the QC were used and 94% if the cutoffs were adjusted for previous antibiotic therapy. In all six patients whose clinical diagnoses of pneumonia were considered definite, both QC of ETA and QC of BAL were positive; however, standard semiQC of ETA showed comparable results in this group. On the contrary, in the 12 patients whose clinical diagnoses were uncertain, QC of BAL and ETA were negative in ten patients and in five (50%) of these, pneumonia was eventually excluded. Semiquantitative cultures of ETA were positive in all these patients. Five (28%) patients experienced a decrease in PaO2/FiO2 (> 15% of previous value) 2 hours after BAL, and in three (17%) this derangement persisted for 24 hours. These data suggest that BAL may be hazardous in mechanically ventilated trauma patients and that its use should be restricted to patients in whom the diagnosis is in doubt.