We performed a secondary analysis of data from a multicenter, prospective cohort study in 10 Italian ICUs. AKI was defined by renal SOFA score (creatinine>3.5mg/dL or urine output (UO) <500mL/d). Oliguria was defined as a UO <500mL/d. Mean fluid balance (MFB) and mean urine volume (MUV) were calculated as the arithmetic mean of all daily values. Use of diuretics was noted daily. To assess the impact of MFB and MUV on mortality of AKI patients, multivariable analysis was performed by Cox regression.Of the 601 included patients, 132 had AKI during ICU stay and the mortality in this group was 50%. Non-surviving AKI patients presented higher MFB (1.31 +/- 1.24 versus 0.17 +/- 0.72 L/day; P<0.001) and lower MUV (1.28 +/- 0.90 versus 2.35 +/- 0.98 L/day; P<0.001) as compared to survivors. In the multivariate analysis, MFB (adjusted hazard ratio (HR) 1.67 per L/day, 95%CI 1.33-2.09; <0.001) and MUV (adjusted HR 0.47 per L/day, 95%CI 0.33-0.67; <0.001) remained independent risk factors for 28-day mortality after adjustment for age, gender, diabetes, hypertension, diuretic use, non-renal SOFA and sepsis. Diuretic use was associated with better survival in this population (adjusted HR 0.25, 95%CI 0.12-0.52; <0.001).In this multicenter ICU study, a higher fluid balance and a lower urine volume were both important factors associated with 28-day mortality of AKI patients.