Introduction: The population is aging and older adults comprise the majority of patients in intensive care units. Colistin (COL) has been reintroduced to treat increasingly common resistant Gram-negative bacterial infections. Our study aims to investigate the factors affecting colistin nephrotoxicity in the general population and geriatric age group. Materials and Method: This retrospective study included 170 patients, 116 (68.2%) of which were in the geriatric group (age ≥65). Acute renal failure was evaluated using the RIFLE score. Firstly, factors associated with COL nephrotoxicity in the general population were investigated. Then, risk factors for COL nephrotoxicity were evaluated in the geriatric patient group. Results: Advanced age (odds ratio [OR]=1.043; 95% confidence interval [CI]: 1.018-1.068; p=0.001) and initial serum creatinine level (OR=23.122; 95% CI: 3.123-171.217; p=0.002) were found to be independent risk factors associated with nephrotoxicity. In the evaluation of the geriatric population-based on nephrotoxicity, the initial serum urea and creatinine levels, immunosuppression, and overall mortality rates were found to be statistically significant in the group with nephrotoxicity (p<0.05). Initial serum creatinine level (OR=22.48; 95% CI: 2.835-178.426; p=0.003) and concomitant nephrotoxic agent use (OR=2.516; 95% CI: 1.275-4.963; p=0.008) were independent risk factors associated with nephrotoxicity in geriatric patients. Conclusion: Advanced age was found to be a risk factor for COL nephrotoxicity. Caution should be exercised especially in geriatric patients who have initial serum creatinine levels close to the upper limit, concomitant use of nephrotoxic drugs should be avoided and if possible, evaluation should be made in terms of non-COL treatment options in these patients.