Background: Risk, Injury, Failure, Loss, and End-Stage (RIFLE) criteria have been proposed as a standard definition of acute kidney injury (AKI). The most severe form of AKI, class F AKI, can be defined by either severe oliguria or a 3-fold increase in serum creatinine concentrations. We hypothesized that the outcomes of patients with these 2 alternative criteria of severe AKI were different. Methods: A prospective cohort study was conducted of all patients attaining RIFLE class F AKI during a 12-month period in a tertiary critical care facility. Results: Among a total of 2,379 critical care admissions, 129 (5.4%) fulfilled the serum creatinine criteria without oliguria (RIFLE class F) and 99 (4.2%) fulfilled oliguric (RIFLE class F) AKI criteria. Patients with oliguric AKI suffered a more severe disease process than nonoliguric AKI. Oliguric AKI was associated with a significantly higher risk of requiring acute dialysis (70.7 vs. 22.4%, p = 0.001), long-term dialysis >90 days (15 vs. 1.9%, p = 0.006), and hospital mortality (adjusted hazard ratio 3.33, 95% confidence interval, p = 0.001) than nonoliguric AKI. Conclusions: Oliguric RIFLE class F AKI is a more severe form of AKI than nonoliguric class F AKI. These 2 forms of AKI should be considered separately when AKI is evaluated in a clinical trial.