Background. Glomerular cellular crescents consist of epithelial cells and macrophages, which can undergo an irreversible process of fibrous organization. However, the origin of the fibroblast-type cells that mediate this fibrous organization is unclear. Methods. This study examined glomerular epithelial myofibroblast transdifferentiation (GEMT) in the formation and evolution of glomerular crescents in two distinct rat models of glomerulonephritis: 5/6 nephrectomy and antiglomerular basement membrane (GBM) disease. Results. Early in the course of both disease models, and prior to crescent formation, immunohistochem-istry staining and in-situ hybridization demonstrated de novo expression of alpha-smooth-muscle actin (α-SMA), a marker of smooth muscle cells and myofib-roblasts, by glomerular parietal epithelial cells (GPEC). The expression of α-SMA by GPEC was accompanied by a loss of E-cadherin staining, a marker of epithelial cells. At this early stage of GEMT, ultra-structural studies identified the presence of character-istic actin microfilaments and dense bodies within GPEC which retained a normal epithelial morphology with apical-basal polarity and microvilli. A late stage of transdifferentiation was seen in fibrocellular cres-cents. In this case, GPEC attached to intact segments of the capsular basement membrane contained large bundles of actin microfilaments throughout the cell, and this was accompanied by a loss of polarity, micro-villi, and tight junctions. There was a significant cor-relation between the presence of α-SMA+ GPEC and glomerular crescent formation. Cellular crescents con-tained small numbers of α-SMA+ myofibroblasts. These cells become the dominant population in fibro-cellular crescents, which was associated with marked local proliferation. Relatively few α-SMA+ myofibro-blasts remained in fibrotic/organizing crescents. Most cells within cellular and fibrocellular crescents expressed transforming growth factor-β(TGF-β) and basic fibroblast growth factor (FGF-2), suggesting that these growth factors may regulate this GEMT process during the evolution of glomerular crescents. Conclusions. This study provides the first phenotypic and morphological evidence that glomerular epithelial-myofibroblast transdifferentiation participates in the formation and evolution of glomerular crescents.