The glomerular mesangial cell may have several important beneficial functions in the normal glomerulus. These include the production of growth factors to allow normal cell turnover, the provision of structural support for the capillaries via the production of mesangial matrix, and the modulation of glomerular hemodynamics via their contractile properties. However, in various types of glomerular injury, the mesangial cell may acquire characteristics of a "myofibroblast", which may in fact be injurious to the glomerulus. These "activated" mesangial cells can be shown to be proliferating by one or more mechanisms that are mediated by platelets and that also involve the local production of platelet-derived growth factor. Like myofibroblasts in other tissues, the mesangial cell acquires smooth muscle cell-like properties, characterized by the de novo expression of alpha-smooth muscle actin, and by the development of fibroblast-like properties, characterized by the production of interstitial collagens in addition to normal mesangial matrix constituents. Identifying therapeutic strategies that prevent this phenotypic modulation of the mesangial cell may provide new ways to treat golmerular diseases.