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Introduction: Acute kidney injury (AKI) is a known risk factor for mortality in critically ill patients. RIFLE criteria aid in staging patients with AKI, and are a predictor of intensive care unit (ICU) outcome. There is limited literature comparing the outcomes of surgical ICU (SICU) patients with AKI who meet RIFLE criteria to those with renal insufficiency (RI) who do not meet RIFLE criteria. Hypothesis: Patients with AKI who meet RIFLE criteria during a SICU admission have increased mortality (M) and ICU length of stay (LOS) compared to patients with RI who do not meet RIFLE criteria and to patients without RI. Methods: This is retrospective review using prospectively collected data taken from the ICU’s database, including 2,723 surgical patients admitted between 10/1/05 and 5/31/12 to the 16-bed ICU of a university-affiliated teaching hospital. RIFLE was defined as the incremental change between initial serum creatined (Cr) and peak ICU Cr values: R1 (1.5-2.0x); R2 (2.0-3.0x); R3 (>3.0x). Other subgroups included patients with RI on ICU admission who did not subsequently meet RIFLE criteria: RI-1 (peak Cr 1.5-1.9); RI-2 (Cr 2.0-2.9); RI-3 (Cr 3.0+); patients without RI (peak Cr <1.5 and no RIFLE criteria); ESRD. Primary outcomes included prolonged LOS (PLOS, 4.0+ days) and M. Multivariate analysis for M included APACHE IV predicted M and ICU LOS. Results: R1 R2 R3 RI-1 RI-2 RI-3 ESRD No RIN 106 46 27 190 104 54 69 2,127PLOS (%) 48.1 52.2 85.2 16.3 26.0 27.8 10.1 10.5M (%) 17.0 43.5 40.7 8.4 16.3 24.1 7.2 3.1Only R1, R2 and R3 were independently associated with increased risk of M: (OR, 95% CI) 3.50 (2.05-5.97); 13.68 (7.46-25.11); 11.55 (5.27-25.32), p<0.0001 for each, respectively, as well as increased risk of PLOS (p<0.0001 for each). Conclusions: In a large single-center cohort of SICU patients, those with AKI who met RIFLE criteria had increased M and PLOS compared to patients without RI who did not meet RIFLE criteria, patients with ESRD, and patients without RI.