Acute kidney injury (AKI) is a common condition with significant associated morbidity and mortality. The insensitivity and non-specificity of traditional markers of renal dysfunction prevent timely diagnosis, estimation of the severity of renal injury, and the administration of possible therapeutic agents. Here, we determine the prognostic ability of urinary liver-type fatty acid-binding protein (L-FABP), and further characterize its sensitivity and specificity as a biomarker of AKI. Initial western blot studies found increased urinary L-FABP in patients with confirmed AKI. A more extensive cross-sectional study found significant increases in urinary L-FABP, normalized to urinary creatinine, in 92 patients with established AKI compared with 62 patients without clinical evidence of AKI. In hospitalized patients, the diagnostic performance of urinary L-FABP for AKI, assessed by the area under the receiver operating characteristic curve, was 0.93. This compares favorably with other established biomarkers of AKI such as kidney injury molecule-1, neutrophil gelatinase-associated lipocalin, N-acetyl-beta-glucosaminidase, and interleukin-18. Our study shows that age-adjusted urinary L-FABP levels were significantly higher in patients with poor outcome, defined as the requirement for renal replacement therapy or the composite end point of death or renal replacement therapy.