PURPOSE:To evaluate the outcomes of the patients who were infected with colistin-only-susceptible (COS) Acinetobacter baumannii and treated with either colistin monotherapy or colistin combined therapy. MATERIALS AND METHODS:This retrospective case-control study was conducted in the training and research hospital with an 800 beds between August 2008 and December 2011. The patients, who were infected with COS A. baumannii and received either colistin monotherapy or colistin combined therapy, were included into the study. RESULTS:In total, 51 patients fulfilling study criteria were evaluated. Colistin monotherapy was found effective as much as colistin combined therapy in terms of clinical and microbiological responses in patients with ventilator associated pneumonia (VAP) and also in patients with blood stream infections. CONCLUSION:Although there is no randomised controlled study yet, colistin monotherapy and colistin combined therapy are likely to achieve similar treatment responses rates. Heteroresistant strains can emerge in patients who receive colistin monotherapy.