The identification of microorganisms causing ventilator-associated pneumonia (VAP) is important for formulating appropriate therapies. In this study, we report the incidence, etiology, and antibiotic resistance patterns of Gram-negative microorganisms isolated from patients diagnosed with VAP in our medical-surgical intensive care unit (ICU) during the years 2004-2006. VAP was diagnosed by using the clinical criteria of the Centers for Disease Control and Prevention. Antibiotic resistance patterns of isolated microorganisms were defined by standard methods. The VAP incidence rate was 22.6/1,000 ventilator days. The most frequently isolated pathogens were Acinetobacter spp., methicillin-resistant Staphylococcus aureus, and Pseudomonas aeruginosa. Ninety percent of Acinetobacter spp. isolates were resistant to ceftazidime, 64% to imipenem, and 80% to ciprofloxacin. Fifty-nine percent of P. aeruginosa isolates were resistant to ceftazidime, 32% to imipenem, and 62% to ciprofloxacin. Cefoperazone-sulbactam was the most active agent against Acinetobacter spp. In conclusion, the incidence of VAP and the prevalence of multidrug-resistant microorganisms are quite high in our ICU. Comparison of the resistance rates of isolates demonstrates that certain antibiotic agents are more effective than others.