Polymyxins have recently again become important because of multidrug-resistant (MDR) gram-negative pathogens. The aim of this study was to evaluate the clinical and microbiological efficacy and toxicity of different dosages of colistin in patients infected with MDR microorganisms that were sensitive only to colistin. The study was conducted in the 1,200-bed Ankara Numune Training and Research Hospital. Patients with normal renal function who received colistin for 48 h or more were retrospectively evaluated. Clinical response was defined as resolution of fever and clinical and laboratory findings. Microbiological response was defined as bacteriological eradication from the infection site. Nephrotoxicity was defined as at least two consecutive serum creatinine measurements with an increase of 0.5 mg/dl from baseline at least 24 h apart after 2 or more days of colistin therapy. Twenty-four patients were included in the study: total clinical response was obtained in 17 of 24 (70.8 %) patients and microbiological response in 15 of 24 (62.5 %) patients. Patients were grouped according to colistin dosage of 3 × 1 million units (MU) versus 3 × 2 MU. Clinical response rates were 69.2 % and 72.7 %, respectively (p = 0.65). Microbiological response rate was similar (p = 0.62). Nephrotoxicity was revealed in 1 of 13 patients (7.7 %) for the 3 × 1 MU group and 2 of 11 patients (18.2 %) in the 3 × 2 MU group (p = 0.57). The nephrotoxicity rate was greater with higher dosages of colistin, but the difference was not statistically significant. Renal function of patients receiving higher dosages of colistin should be more closely monitored.