Background. Acute kidney injury (AKI) is a recognized complication of cardiac surgery; however, the variability in costs and outcomes reported are due, in part, to different criteria for diagnosing and classifying AKI. We determined costs, resource use and mortality rate of patients. We used the serum creatinine component of the RIFLE system to classify AKI. Methods.Aretrospectivecohortstudywasconductedfrom theelectronicdatarepositoryattheUniversityofPittsburgh Medical Center of patients who underwent cardiac surgery and had an elevation (≥0.5mg/dl) of serum creatinine post- operatively. Data were compared to age- and APACHE III- matched controls. Cost, mortality and resource use of AKI patients were determined postoperatively for each of the three RIFLE classes on the basis of changes in serum crea- tinine. Results. Of the 3741 admissions, 258 (6.9%) had AKI and wereclassifiedasRIFLE-R138(3.7%),RIFLE-I70(1.9%) andRIFLE-F50(1.3%).Totalanddepartmentallevelcosts, length of stay (LOS) and requirement for renal replacement therapy (RRT) were higher in AKI patients compared to controls. Statistically significant differences in all costs, mortality rate and requirement for RRT were seen in the patients stratified into RIFLE-R, RIFLE-I and RIFLE-F. Even patients with the smallest change in serum creatinine, namely RIFLE-R, had a 2.2-fold greater mortality, a 1.6- fold increase in ICU LOS and 1.6-fold increase in total postoperative costs compared to controls. Discussion. Costs, LOS and mortality are higher in post- operative cardiac surgery patients who develop AKI using RIFLE criteria, and these values increase as AKI severity worsens.