Purpose: It is controversial whether all critically ill patients with risk, injury, failure, loss, and endstage renal failure (RIFLE) F class acute kidney injury (AKI) should receive renal replacement therapy (RRT). We reviewed the outcome of open heart surgery patients with severe RIFLE-F AKI who did not receive RRT. Materials and Methods: We identified all patients with AKI after cardiac surgery over 4 years and obtained baseline characteristics, intraoperative details, and in-hospital outcomes. We analyzed physiologic and biochemical features at RRT initiation or at peak creatinine if no RRT was provided. Results: We reviewed 1504 patients. Of these, 137 (9.1 ) developed postoperative AKI with 71 meeting RIFLE-F criteria and 23 (32.4 of RIFLE-F cases) not receiving RRT. Compared with RRTtreated RIFLE-F patients, ?no-RRT? patients had lower Acute Physiology and Chronic Health Evaluation III scores, less intra-aortic balloon pump requirements, shorter intensive care stay, and a trend toward lower mortality. At peak RIFLE score, their urinary output, arterial pH, and PaO2/ fraction of inspired oxygen ratio were all significantly higher. Their serum creatinine was also higher (304 vs 262 ?mol/L; P = .02). Only 3 RIFLE-F no-RRT patients died in-hospital. Detailed review of cause and mode of death was consistent with non?RRT-preventable deaths. In contrast, 27 patients with RIFLE-R or RIFLE-I class received RRT. Compared with RRT-treated RIFLE-F patients, such RIFLE-R or RIFLE-I treated patients had a more severe presentation and higher mortality (51.8 vs 29.2 ; P = .02). Conclusions: After cardiac surgery, RRT was typically applied to patients with the most severe clinical presentation irrespective of creatinine levels. A RIFLE score?based trigger for RRT is unlikely to improve patient survival.