This study compared the incidence of acute kidney injury in patients treated with polymyxin B or colistin at a large academic medical center using the RIFLE criteria. The results showed that the incidence of acute renal failure was significantly higher with colistin (60.4% vs 41.8%, p=0.02). Multivariate analysis identified colistin use and older age as independent risk factors for increased nephrotoxicity. The study supports closer monitoring of patients receiving these drugs and indicates a need for further research to understand differences in nephrotoxicity between polymyxin B and colistin.