Introduction: Acinetobacterbaumannii (A. baumannii) is a Gram-negative coccobacillus and is a frequent cause of hospital-acquired infections. Because some strains of A. baumannii are resistant to many antibiotics (i.e., extensively drug resistant A. baumannii, or XDRAB), selecting antibiotics to treat infected patients is challenging. Clinical outcomes in critically ill patients with XDRAB infections are poor. In this study, we evaluated the clinical effectiveness of colistin as monotherapy and in combination with other antibiotics. Methods: 94 critically ill patients clinical effectiveness of treating XDRAB pulmonary infections with colistin, either in monotherapy or in combination with tigecycline, meropenem, or both. Clinical and microbiological data were obtained from patient records. We included patients suffering from XDRAB ventilation-associated pneumonia (VAP) or VAP with bacteremia. Results: The mean age of the patients was 53.3 years (standard deviation =23.7 years) and the mean APACHE II score was 22.7 (SD =7.1). Respiratory tract infections and bacteremia were found in 84% and 16% of patients, respectively. Half (51%) of patients achieved microbiological clearance. The median ICU stay was 29 days and the mean mechanical ventilation (MV) duration was 21 days. MV duration and ICU length of stay were lower in the group of patients treated with colistin and meropenem than in those treated with colistin alone. Mortality was significantly lower in patients who received (colistin and tigecycline 30%) than in those who were treated with monotherapy (75%). Conclusions: Colistin-based combination treatment regimens mainly with tigecycline or with tigecycline and meropenem are potentially more effective for the treatment of XDRAB induced VAP than colistinmonotherapy.