Fibrosis is the response of heart and other organs to injuries. Excessive fibrosis can cause organ dysfunction or even failure. Transforming-growth factor (TGF)-beta is a cytokine that induces fibroblast proliferation and increases the synthesis of a number of extracellular matrix proteins including collagens. Decorin (DCN) is a natural antagonist of TGF-beta. In the current study, we investigated the potential antifibrotic effects of DCN gene delivery by a recombinant adeno-associated viral (rAAV) vector to inhibit cardiac fibrosis in old, spontaneously hypertensive rats (SHRs), which develop severe cardiac and kidney fibrosis if without intervention. The rAAV-DCN vector was injected (at a dose of 1 x 10(11) vector genomes) via the tail vein into 5-month-old male SHRs, resulting in persistent, stable expression of DCN (up to 16 weeks). rAAV-DCN treatment significantly reduced collagen content and fibrosis in the heart and attenuated cardiomyocyte hypertrophy. Hemodynamics data at 16 weeks showed that DCN gene delivery induced a significant increase in left ventricular end-systolic pressure and maximal-minimal rate of pressure increase (+/-dp/dt(max)), but a decrease in left ventricular end-diastolic pressure (p < 0.05), compared with those of control animals. The expression of TGF-beta and alpha-smooth muscle actin, and the phosphorylation levels of Smad2 and p38 MAPK, were markedly reduced by rAAV-DCN treatment as compared with the controls. Thus, these results suggest that rAAV-mediated DCN overexpression led to the inhibition of hypertension-induced cardiac fibrosis and hypertrophy and improved cardiac function, and therefore may have therapeutic potential for organ fibrosis.