The incidence of acute kidney injury (AKI) has been reported to vary between 17% and 95% post–orthotopic liver transplantation. This variability may be related to the absence of a uniform definition of AKI in this setting. The purpose of this study was to identify the degree of AKI that is associated with long-term adverse outcome. Furthermore, to determine the best definition (for use in future studies) of AKI not requiring dialysis in post–liver transplant patients, we retrospectively reviewed the effect of 3 definitions of AKI post–orthotopic liver transplantation on renal and patient outcome between 1997 and 2005. We compared patients with AKI to a control group without AKI by each definition. AKI was defined in 3 groups as an acute rise in serum creatinine, from the pretransplant baseline, of >0.5 mg/dL, >1.0 mg/dL, or >50% above baseline to a value above 2 mg/dL. In all groups, the glomerular filtration rate was significantly lower at both 1 and 2 years post-transplant. Patient survival was worse in all groups. Graft survival was worse in all groups. The incidence of AKI was highest in the group with a rise in creatinine of >0.5 mg/dL (78%) and lowest in patients with a rise in creatinine of >50% above 2.0 mg/dL (14%). Even mild AKI, defined as a rise in serum creatinine of >0.5 mg/dL, was associated with reduced patient and graft survival. However, in comparison with the other definitions, the definition of AKI with the greatest impact on patient's outcome post–liver transplant was a rise in serum creatinine of >50% above baseline to >2 mg/dL. Liver Transpl 15:475–483, 2009. © 2009 AASLD.