Introduction: Colistin use has increased over the last ten years because of multidrug-resistant microorganisms. The aim of this study was to compare the clinical and microbiological efficacy of colistin alone or in combination with sulbactam or carbapenem in the treatment of ventilator-associated pneumonia (VAP) due to multidrug-resistant (MDR) and extremely drug-resistant (XDR) A. baumannii.