OBJECTIVE: To assess the influence of antibiotics on the value of various cytological parameters, and their combinations, in diagnosing ventilator-associated pneumonia (VAP). DESIGN: Prospective study. SETTING: The general intensive care unit (17 beds) of the University Hospital Maastricht. PATIENTS: Three hundred and thirty-five episodes of clinically suspected VAP (defined by the clinical and radiological criteria previously described by Bonten et al.) in 282 patients were studied. INTERVENTIONS: No additional interventions were conducted. MEASUREMENTS AND RESULTS: Bronchoalveolar lavage fluid cytology included a total cell count per millilitre, differential cell count and the percentage of infected cells (cells containing phagocytised organisms). Antibiotic therapy from 72 h prior to lavage was recorded. Areas under the curve (AUCs) of receiver operating characteristic curves were calculated for various cytological parameters and their combinations, in patients with and without antibiotic therapy. In 126 episodes (37.6%) in 106 patients, VAP was confirmed. There was no difference in AUCs between patients with and without antibiotic therapy for any parameter studied. The most prominent AUCs were (for patient groups with and without antibiotics combined): total cell count, 0.65; percentage polymorphonuclear neutrophils, 0.71; and percentage infected cells, 0.90. The combination of percentage infected cells with any other cytological parameter did not increase the AUC. CONCLUSION: Antibiotic therapy did not influence the predictive value of the percentage infected cells in BALF in diagnosing VAP.