© 2017, Springer International Publishing AG.Objective: Most significant side effect of colistin therapy which is used for the treatment of multi-drug resistant Gram-negative infections is nephrotoxicity. Our aim was to investigate the differences of colistin nephrotoxicity between the geriatric age group (≥65 years) and the younger age group (<65 years) in critically ill medical intensive care unit (ICU) patients. Material and method: The medical records of the 76 patients who were taken colistin therapy due to multi-resistant Gram-negative infections between January 2010 and June 2014 in the our medical ICU were retrospectively investigated. Demographic characteristics, reasons for colistin use, daily colistin dose, duration of colistin use were recorded. Colistin-dependent renal dysfunction was evaluated according to the risk, injury, failure, loss and end-stage renal failure (RIFLE) criterias. Results: The median age of the patients was 65 (65.8% male). Nephrotoxicity was developed in 36 (47.4%) patients. Thirty-nine (51.3%) patients were in geriatric age group, 37 (48.7%) were in younger age group. In geriatric age group, the rates of male gender (53.8 vs 78.4%, p = 0.031), pulmonary (48.7 vs 16.2%, p = 0.003) and cardiac diseases (71.8 vs 29.7%, p < 0.001), post-nephrotoxicity BUN levels (p = 0.023) and urine output during nephrotoxicity (p = 0.016) were higher than younger age group. Nephrotoxicity was developed in 22 (56.4%) patients of geriatric age group, and in 14 (37.8%) patients in younger age group (p = 0.115). The presence of cardiac disease, renal pathology and high creatinin value on admission, daily amount of colistin per body mass, total amount of colistin, use of colistin for pulmonary infection, use of amphotericin and vasopressor on admission were found as risk factors for colistin nephrotoxicity development in all study group; the daily amount of colistin per body mass (risk ratio:0.41; 95% CI 0.19–0.89) and vasopressor use during hospitalization were found independent risk factors (risk ratio:13.54; 95% CI 2.21–83.09). Conclusion: In our study, in geriatric patient group colistin nephrotoxicity was not different from the younger age group. In the ICU, the age for nephrotoxicity does not appear to be a point to be considered for the initiation of colistin.