Objective: To investigate whether there was a difference in mortality, clinical response and bacterial eradication between colistin monotherapy and colistin combination therapies for the treatment of nosocomial pneumonia/ventilator-associated pneumonia (VAP) caused by Acinetobacter baumannii in a medical intensive care unit (ICU). Methods: This retrospective, observational and single-centre study included all patients who were in the medical ICU of Gazi University Medical Faculty Hospital and diagnosed with nosocomial pneumonia/VAP caused by A. baumannii between January 2009 and September 2014. Results: The median age of the 134 patients was 68 years and 53.3% were male. The most common causes of admission were respiratory insufficiency (66.7%) and sepsis/septic shock (54.8%). In patients with nosocomial pneumonia/VAP caused by A. baumannii, on median day 5 of admission, colistin monotherapy was used in 23 (21.6%) patients, a carbapenem combination was used in 80 (59.7%) patients, sulbactam-ampicillin combination was used in 42 (31.4%) patients, tigecycline combination was used in 26 (19.4%) patients, and sulbactam-cefoperazone combination was used in 17 (12.7%) patients. Median ICU stay of the patients was 15.5 days, and 112 (83.6%) patients died. Colistin monotherapy and combination therapies had no superiority over each other in clinical response for the treatment of A. baumannii-associated nosocomial pneumonia/VAP. Mortality was found to be higher in patients receiving the colistin-carbapenem combination (64.3% v. 36.4%, p=0.016). Discharge/day-of-death Sequential Organ Failure Assessment score (odds ratio (OR) 2.017, 95% confidence interval (CI) 1.330-3.061) and vasopressor use (OR 9.014, 95% CI 1.360-59.464) were independent risk factors for ICU mortality. Conclusion: Colistin monotherapy and combination therapies have no superiority over each other for clinical response in the treatment of nosocomial pneumonia/VAP caused by multidrug-resistant A. baumannii. Colistin-SAM was associated with improved microbiological eradication and colistin-carbapenem combination was associated with increased mortality.