<b><i>Background/Aims:</i></b> Chronic kidney disease is characterized by accumulation of extracellular matrix in the tubulointerstitial area. Fibroblasts are the main matrix-producing cells. One source of activated fibroblasts is the epithelial mesenchymal transition (EMT). In cultured tubular epithelial cells, transforming growth factor-β (TGF-β₁) induced Gremlin production associated with EMT phenotypic changes, and therefore Gremlin has been proposed as a downstream TGF-β₁ mediator. Gremlin is a developmental gene upregulated in chronic kidney diseases associated with matrix accumulation, but its direct role in the modulation of renal fibrosis and its relation with TGF-β has not been investigated<i>. </i><b><i>Methods:</i></b> Murine renal fibroblasts and human tubular epithelial cells were studied. Renal fibrosis was determined by evaluation of key profibrotic factors, extracellular matrix proteins (ECM) and EMT markers by Western blot/confocal microscopy or real-time PCR. Endogenous Gremlin was targeted with small interfering RNA. <b><i>Results:</i></b> In murine fibroblasts, stimulation with recombinant Gremlin upregulated profibrotic genes, such as TGF-β₁, and augmented the production of ECM proteins, including type I collagen. The blockade of endogenous Gremlin with small interfering RNA inhibited TGF-β₁-induced ECM upregulation. In tubular epithelial cells Gremlin also increased profibrotic genes and caused EMT changes: phenotypic modulation to myofibroblast-like morphology, loss of epithelial markers and in-duction of mesenchymal markers. Moreover, Gremlin gene silencing inhibited TGF-β₁-induced EMT changes. <b><i>Conclusions:</i></b> Gremlin directly activates profibrotic events in cul-tured renal fibroblasts and tubular epithelial cells. Moreover, endogenous Gremlin blockade inhibited TGF-β-mediated matrix production and EMT, suggesting that Gremlin could be a novel therapeutic target for renal fibrosis.