We evaluated the diagnostic yield of bilateral bronchoalveolar lavage (BAL) in patients with acute respiratory distress syndrome (ARDS) with suspected ventilator-associated pneumonia (VAP) and compared BAL results from contralateral sites. Ninety-four ARDS patients with suspected VAP underwent 172 bronchoscopies (344 BALs). BAL was processed for quantitative cultures, total cell count and subjected to microscopic analysis for cell differential, presence of intracellular organisms (ICO), and Gram stain. The diagnostic threshold for VAP was a growth of >= 10(4) cfu/ml in BAL culture. Most episodes (68%) had bilateral insignificant bacterial growth. Forty (43%) patients had one or more episodes of VAP. Thirty-three of the 55 (60%) positive bronchoscopies had significant growth in only one side, 18 were right BAL, and 15 were left BAL. Episodes with bilateral significant growth were more likely to be polymicrobial, to have a bacterial growth >= 10(5) cfu/ml in the BAL, and to possess a higher percentage of neutrophils and ICO. Among 65 microorganisms recovered in significant concentration, Pseudomonas aeruginosa occurred in 43% and S. aureus in 15%. Overall, Gram stain had a sensitivity of 54% and a specificity of 87%; and Giemsa stain (> 2% ICO) had a sensitivity of 46% and a specificity of 93%. Antibiotic treatment was modified by the results of BAL cultures in 50 (91%) episodes of pneumonia. In patients with ARDS and suspected VAP, bilateral BAL quantitative bacterial cultures had significant growth on one side only in 19% and in both sides in 13%.