Traditional cutoff values of serum creatinine considered to define postoperative acute renal failure have been challenged recently. In a previous investigation we demonstrated that minimal changes in serum creatinine concentration were associated with a substantial decrease in survival after cardiac surgery. In this investigation, we assessed the impact of minimal absolute increases in serum creatinine in a second institution, and we analyzed whether relative changes, as in the RIFLE classification and, partially, in Acute Kidney Injury Network (AKIN) classification, confer a different prognostic potential.