BACKGROUND Ischemia/reperfusion (I/R) injury is closely associated with tissue damage in various organs, as well as in kidney transplants. Erythropoietin (EPO) has been shown to have a cytoprotective effect against hypoxia. We examined the effect of EPO against renal I/R injury and the underlying mechanism. METHODS Human umbilical vein endothelial cells and human renal proximal tubular epithelial cells were cultured under hypoxic conditions with various EPO concentrations at 37 degrees C and examined the mechanism of cell proliferation by EPO. Moreover, to demonstrate the renoprotective effect in vivo, we treated Sprague-Dawley rats with 100 IU/kg EPO every 2 days for 2 weeks (a total of 6 doses). One day after the last injection, the operations to produce renal I/R injury (bilateral renal occlusion for 60 min) were done, and rats were killed at the end of the reperfusion period (24 hr and 72 hr after reperfusion began). RESULTS First, we demonstrated in vitro that EPO increased hypoxia inducible factor-1alpha (HIF-1alpha) expression and stimulated proliferation of both cells under hypoxic conditions. Next, we demonstrated in vivo that EPO treatment increased the number of HIF-1alpha-positive cells, and markedly induced the expression of vascular endothelial growth factor messenger RNA. Using pimonidazole, a molecular probe that detects hypoxia, we found that EPO markedly attenuated tubular hypoxia, and reduced the number of terminal transferase dUTP nick end labeling-positive apoptotic cells and alpha-smooth muscle actin-positive interstitial cells. CONCLUSIONS We suggested a novel HIF-1alpha induction pathway by EPO under hypoxic conditions. Thus, EPO may protect the kidneys against ischemia reperfusion injury by activating HIF-1alpha.