Recently, colistin has become a salvage therapy in the treatment of serious Intensive Care Unit infections owing to the emergence of extensively drug-resistant (XDR) bacterial isolates. This study aimed to show the effectiveness of colistin in critically ill patients with renal failure. A prospective case-control study of 94 patients admitted to medical intensive care units of a university hospital from December 2008 to June 2010 was conducted. All patients had infections with XDR Acinetobacter baumannii or Pseudomonas aeruginosa and received colistin. Cases comprised 39 patients with chronic renal failure (CRF) and controls were other patients without CRF. Apart from the male dominancy in the CRF group, there was no statistical difference between the two groups regarding demographic characteristics, Acute Physiology and Chronic Health Evaluation (APACHE) II score, and site and type of infection. In patients who completed colistin therapy, bacteriological cure was seen in 87% of patients with CRF and 95% of patients without CRF (P = 0.890). Mortality in patients with CRF was similar to that in patients without CRF (44% and 42%, respectively) (P = 0.999). Nephrotoxicity developed in 23.6% of patients in the control group. Concomitant nephrotoxic agents and total defined daily dose of colistin did not affect the development of nephrotoxicity. The mortality rate was 38% in patients with nephrotoxicity, similar to the mortality rate in patients without nephrotoxicity (36%) (P = 0.999). In conclusion, in critically ill patients with CRF, colistin therapy, although used at a reduced dosage, was as effective as in patients without CRF. © 2011 Elsevier B.V. and the International Society of Chemotherapy. All rights reserved.