Mesangial cell proliferation constitutes a frequent finding in a number of glomerular diseases that progress to glomerular sclerosis. The factors responsible for mesangial cell growth regulation in vivo are ill defined. However, cell culture data indicate that an array of mediators may have mitogenic or antimitogenic effects on these cells. This brief review discusses the relevance of selected factors such as platelet-derived growth factor (PDGF) in this context. In vitro data indicate that PDGF is one of the most potent mesangial cell mitogens, that it may have autoregulatory properties, and that it may represent the final common pathway for a number of other mitogenic peptides. In contrast to PDGF, the relevance of inflammatory cytokines such as interleukin-1 (IL-1), tumor necrosis factor (TNF) or interleukin-6 (IL-6) for mesangial cell proliferation is less evident, with growth-inhibitory to weakly growth-promoting effects on mesangial cells. Transforming growth factor β (TGFβ) appears to be unique in that it has a concentration-dependent mitogenic or antimitogenic effect on mesangial cells. Prostaglandins may also have variable effects, ranging from mitogenic (PGE_2α) to growth inhibitory (PGI₂, PGF₂, TxA₂). These data support the notion that mesangial cell growth in vivo is regulated by a complex network of synergistic or antagonistic growth factors. The relative importance of each of these growth factors in the in vivo situation will have to be elucidated by future studies using specific receptor antagonists or neutralizing antibodies.