Background/aim: Colistin is used as a salvage therapy for multidrug-resistant and extremely drug-resistant gram-negative bacterial infections. Our aim was to evaluate colistin efficiency and toxicity in the treatment of these resistant gram-negative bacterial infections. Materials and methods: This is a retrospective study carried out in a tertiary care hospital during 2011-2013. Study data were collected from the medical records and consultations of the infectious diseases clinic. Results: The study group included 158 patients with nosocomial infections and 136 (86.1%) of them were hospitalized in the ICU. Respiratory tract infections were the most commonly observed ones (n = 103, 65.2%). The most frequently isolated microorganism was Acinetobacter baumannii (72.2%). A total of 98 (62.0%) patients had clinical cure. There was no statistically significant difference between monotherapy (n = 3/6, 50.0%) and combination therapies (n = 95/152, 62.5%) according to clinical response. Underlying ultimately fatal disease, previous renal disease, and total parenteral nutrition were independent risk factors for poor clinical response. Nephrotoxicity developed in 80 (50.6%) patients and clinical cure was statistically unrelated with nephrotoxicity. Conclusion: Colistin may be used as an effective agent for multidrug-resistant and extremely drug-resistant gram-negative bacterial infections with close monitoring of renal functions, especially for older and critically ill patients.