BACKGROUND The emergence of infections related to multidrug-resistant Gram-negative bacilli (MDR-GNB) reintroduced the use of colistin, an antibiotic that was previously abandoned due to adverse effects. However, because of its limited use in neonatal intensive care units, there is very little data about the effectiveness and safety of colistin in children and newborns. In this study, which will be the largest case study in the literature, we aimed to evaluate the effectiveness and safety of colistin in full-term and preterm newborns. MATERIAL AND METHODS The study included patients admitted into 2 level 3 neonatal intensive care units between January 2013 and June 2015. The medical records of patients diagnosed with sepsis, meningitis, pneumonia, and urinary tract infection based on the diagnostic culture results and treated with colistin were analyzed retrospectively. The patients whose infections were not verified were excluded from the study. RESULTS The study included 65 patients (18 term, 47 preterm). The most frequently isolated pathogens were Klebsiella pneumoniae and Acinetobacter baumannii followed by Pseudomonas aeruginosa and Enterobacter cloacae. Mean colistin treatment time was 15±3.5 days. All patients treated with colistin were being treated with at least 1 other antibiotic. While a complete clinical response was achieved in 51 (72.3%) patients, 14 (21.5%) patients died during treatment. Four (7.7%) patients died during as a result of another infection. Three patients developed renal toxicity, another 3 patients had seizures, and apnea was observed in 3 patients. CONCLUSIONS Colistin was found to be effective and safe for treatment of MDR-GNB infections in preterms and infants with very low birth weight. Given the severity of the infection, the adverse effects of colistin were at acceptable levels.