Increasing evidence suggests that chronic kidney disease may develop following acute kidney injury and that this may be due, in part, to hypoxia-related phenomena. Hypoxia-inducible factor (HIF) is stabilized in hypoxic conditions and regulates multiple signaling pathways that could contribute to renal fibrosis. As transforming growth factor (TGF)-beta is known to mediate renal fibrosis, we proposed a profibrotic role for cross talk between the TGF-beta1 and HIF-1alpha signaling pathways in kidney cells. Hypoxic incubation increased HIF-1alpha protein expression in cultured human renal tubular epithelial cells and mouse embryonic fibroblasts. TGF-beta1 treatment further increased HIF-1alpha expression in cells treated with hypoxia and also increased HIF-1alpha in normoxic conditions. TGF-beta1 did not increase HIF-1alpha mRNA levels nor decrease the rate of protein degradation, suggesting that it enhances normoxic HIF-1alpha translation. TGF-beta receptor (ALK5) kinase activity was required for increased HIF-1alpha expression in response to TGF-beta1, but not to hypoxia. A dominant negative Smad3 decreased the TGF-beta-stimulated reporter activity of a HIF-1alpha-sensitive hypoxia response element. Conversely, a dominant negative HIF-1alpha construct decreased Smad-binding element promoter activity in response to TGF-beta. Finally, blocking HIF-1alpha transcription with a biochemical inhibitor, a dominant negative construct, or gene-specific knockdown decreased basal and TGF-beta1-stimulated type I collagen expression, while HIF-1alpha overexpression increased both. Taken together, our data demonstrate cooperation in signaling between Smad3 and HIF-1alpha and suggest a new paradigm in which HIF-1alpha is necessary for normoxic, TGF-beta1-stimulated renal cell fibrogenesis.