The aim of our study was to assess the new diagnostic criteria of acute kidney injury (AKI) proposed by the Acute Kidney Injury Network (AKIN) in a large cohort of mechanically ventilated patients.This is a prospective observational cohort study enrolling 2783 adult intensive care unit patients under mechanical ventilation (MV) with data on serum creatinine concentration (SCr) in the first 48 hours. The absolute and the relative AKIN diagnostic criteria (changes in SCr ≥ 0.3 mg/dl or ≥ 50% over the first 48 hours of MV, respectively) were analyzed separately. In addition, patients were classified into three groups according to their change in SCr (ΔSCr) over the first day on MV (ΔSCr): group 1, ΔSCr ≤ -0.3 mg/dl; group 2, ΔSCr between -0.3 and +0.29 mg/dl; and group 3, ΔSCr ≥ +0.3 mg/dl). The primary end point was in-hospital mortality, and secondary end points were intensive care unit and hospital length of stay, and duration of MV.Of 2783 patients, 803 (28.8%) had AKI according to both criteria: 431 only absolute (AKI(A)), 362 both relative and absolute (AKI(R+A)), and 10 only relative. The relative criterion identified more patients when baseline SCr (SCr₀) was <0.9 mg/dl and the absolute when SCr₀ was >1.5 mg/dl. The diagnosis of AKI was associated with mortality.Our study confirms the validity of the AKIN criteria in a population of mechanically patients and the criteria's relationship with the baseline SCr.