Endotracheal aspirates (ETA) of 75 patients who required mechanical ventilation in different intensive care units in Istanbul Faculty of Medicine were investigated in this study. The clinical diagnosis of ventilator-associated pneumonia was confirmed microbiologically (quantitative culture and microscopy) for 37 of the 50 patients. Methicillin-resistant Staphylococcus aureus (46.5%), Pseudomonas aeruginosa (18.6%), Acinetobacter spp. (13.9%) were the most frequently isolated microrganisms in these patients. When microbiological results were compared with the clinical ones, the suitable cut off value was determined as 105 CFU/ml for quantitative cultures to discriminate between colonization and infection, and microscopy was considered as an aiding-tool for this discrimination.