In ventilator-dependent patients the management of clinically suspected nosocomial pneumonia is often difficult. A diagnosis of pneumonia is based upon findings including pulmonary infiltrate, fever, leukocytosis, or purulent secretions. By using bronchoscopic techniques, we can obtain bronchoalveolar lavage specimens (BAL) from the affected area of the lung. Neutrophils (PMN), which are important for lung defense, are found in increased numbers in BAL of these patients. We therefore ascertained the phagocytic activity of PMN against bacteria and fungi by microscopic examination. BAL specimens from ten mechanically ventilated patients were evaluated to assess the cellular counts using a Bürker hemocytometer, the differential cell counts by cytospin preparations (MGG stain), and the phagocytic activity of PMN and macrophages using the intracellular bacteria index (ICB) values. Microscopical examination of BAL cells and evaluation of ICB values (cut-off > 5%) were higher in four out of twelve patients and the quantitative assessment of bacteria in PMN cytoplasm on cytospin preparations was found to be useful for the diagnosis of pneumonia. In these patients, pneumonia was suspected (in one patient fungal pneumonia) on the basis of microscopical examination of BAL cells and ICB values and the findings were confirmed later by microbiological cultures. In conclusion, in patients on mechanical ventilation a rapid diagnosis of bacterial or fungal pneumonia can be made using BAL cytology and by ICB values, and this in turn allows appropriate therapy to be initiated at an early stage. However, further studies of neutrophil functions are required to improve our understanding of the increased incidence of pulmonary infections in these patients.