The repeatability (i.e., the variation in repeated measurements of the same quantity) of the protected specimen brush (PSB) with quantitative cultures was assessed in 22 consecutive mechanically ventilated (MV) patients with suspected nosocomial pneumonia. Five PSB samples were collected in the same lung area during the same bronchoscopic procedure and processed for bacteriologic identification and quantitative culture. A laboratory control was also performed in order to assess the in vitro repeatability of the quantitative culture technique. The five PSB always recovered the same microorganisms, indicating a 100% qualitative repeatability for the PSB. Conversely, the quantitative repeatability was somewhat lower since in 59% of the patients the quantitative results varied by more than one log10, which is the minimal precision affordable with quantitative cultures. The distinction between presence or absence of infection based on the 10(3) cfu/ml recommended diagnostic threshold was, however, only moderately affected by the variability of the quantitative results since only three of 22 patients (13.6%) displayed results spread out on each side of the 10(3) cfu/ml break point. Intrasubject variability of quantitative results was not explained by problems with the quantitative culture technique, which proved excellent repeatability in the laboratory. This study indicated that, although the PSB technique with quantitative cultures displays an acceptable level of repeatability, caution is advisable when interpreting PSB results in critically ill patients with suspected pneumonia, especially if one refers to the 10(3) cfu/ml recommended diagnostic threshold and if a decision to treat or to abstain from treating is to be made.