AKI induces upregulation of heme oxygenase 1 (HO-1), which exerts cytoprotective effects and modulates the renal response to injury, suggesting that a biomarker of intrarenal HO-1 activity may be useful. Because HO-1 largely localizes to the endoplasmic reticulum and has no known secretory pathway, it is unclear whether plasma or urinary levels of HO-1 reflect intrarenal HO-1 expression. We measured plasma and urinary levels of HO-1 by ELISA during the induction and/or maintenance phases of four mouse models of AKI: ischemia/reperfusion, glycerol-induced rhabdomyolysis, cisplatin nephrotoxicity, and bilateral ureteral obstruction. In addition, we measured levels of HO-1 mRNA and protein in the renal cortex. Each AKI model increased renal HO-1 gene expression, which corresponded with release of HO-1 into plasma and urine by 4 hours. Over time, the magnitudes of plasma and urinary HO-1 paralleled renal cortical gene expression. AKI and the associated uremia did not seem to affect extrarenal HO-1 gene activity assessed in the liver, lung, and spleen. In iron-challenged, cultured proximal tubule cells, we observed a positive correlation between HO-1 mRNA level and HO-1 release. In humans, 10 patients with AKI demonstrated markedly higher levels of plasma and urine HO-1 levels than 10 critically ill patients without AKI or 20 patients with CKD or ESRD. In summary, these data suggest that plasma and urinary HO-1 levels may serve as biomarkers of AKI and intrarenal HO-1 gene activity.