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Acute kidney injury affects 5% to 7% of all hospitalized patients with a much higher incidence in the critically ill. The Acute Kidney Injury Network proposed a definition in which serum creatinine rises (>0.3 mg/dL) and/or oliguria (<0.5 mL/kg/hr) for a period of 6 hrs are used to detect acute kidney injury. Accurate urine output measurements as well as serum creatinine values from our database were used to detect patients with acute kidney injury and calculate their corresponding mortality risk and length of stay.Retrospective cohort study.Seven intensive care units at a large, academic, tertiary medical center.Adult patients without evidence of end-stage renal disease with more than two creatinine measurements and at least a 6-hr urine output recording who were admitted to the intensive care unit between 2001 and 2007.Medical records of all the patients were reviewed. Demographic information, laboratory results, charted data, discharge diagnoses, physiological data, and patient outcomes were extracted from the Multiparameter Intelligent Monitoring in Intensive Care II database using a SQL query.From 19,677 adult patient records, 14,524 patients met the inclusion criteria. Fifty-seven percent developed acute kidney injury during their intensive care unit stay. Inhospital mortality rates were: 13.9%, 16.4%, 33.8% for acute kidney injury 1, 2, and 3, respectively, compared with only 6.2% in patients without acute kidney injury (p < .0001). After adjusting for multiple covariates, acute kidney injury was associated with increased hospital mortality (odds ratio 1.4 and 1.3 for acute kidney injury 1 and acute kidney injury 2 and 2.5 for acute kidney injury 3; p < .0001). Using multivariate logistic regression, we found that in patients who developed acute kidney injury, urine output alone was a better mortality predictor than creatinine alone or the combination of both.More than 50% of our critically ill patients developed some stage of acute kidney injury resulting in a stagewise increased mortality risk. However, the mortality risk associated with acute kidney injury stages 1 and 2 does not differ significantly. In light of these findings, re-evaluation of the Acute Kidney Injury Network staging criteria should be considered.