Transforming growth factor-beta (TGF-beta) is a cytokine that is important in embryogenesis, development, carcinogenesis, and tissue repair. TGF-beta is unique among cytokines in its widespread actions on the regulation of extracellular matrix. In a model of acute mesangial proliferative glomerulonephritis, it was shown that overproduction of TGF-beta is the cause of pathologic matrix accumulation in the nephritic glomeruli. TGF-beta acted to increase matrix production, inhibit matrix degradation, and modulate matrix receptors in the glomerulonephritic rats. Elevated expression of TGF-beta was also found in other experimental glomerular diseases, including diabetic nephropathy. Studies of humans with glomerulonephritis and diabetic nephropathy also strongly implicated TGF-beta in the pathogenesis of glomerular matrix build-up. Recently, the proteoglycan decorin was shown to neutralize TGF-beta. When injected into glomerulonephritic rats, decorin markedly suppressed pathologic matrix deposition in the glomeruli. Thus, decorin offers hope as a treatment for progressive kidney diseases caused by the overproduction of TGF-beta.