Polymyxins have recently reemerged as a treatment option in response to the increasing number of resistant bacterial infections seen in recent years. Therefore, the current study aimed to determine the rate of and risk factors related to colistin-associated nephrotoxicity. All adult patients who had received colistimethate sodium (CMS) between 2010 and 2012 and met the inclusion criteria were included in the study. RIFLE (Risk, Injury, Failure, Loss of renal function and End stage of renal disease) criteria were used to evaluate nephrotoxicity. Age, sex, underlying diseases presences, daily and total CMS doses, daily blood urea and creatinine levels, as well as concurrent drug use were recorded for each patient. Nephrotoxicity occurred in 48% of patients. There was a significant difference in the baseline serum urea levels of patients who experienced nephrotoxicity and those who did not (P value (P) = 0.015). Furthermore, the multivariate analysis showed that advanced age and concomitant aminoglycoside-class antibiotic use were significantly associated with nephrotoxicity. In conclusion, colistin should be used carefully, and all patients should be monitored closely for renal nephrotoxicity.