Myofibroblasts are metabolically and morphologically distinctive fibroblasts expressing alpha-smooth muscle actin (alpha-SMA), and their activation plays a key role in development of the fibrotic response. In an activated state, myofibroblasts cease to proliferate and start to synthesize large amounts of extracellular component proteins. The expression of alpha-SMA correlates with the activation of myofibroblasts. Decorin, a member of the small leucine-rich proteoglycan gene family, has been implicated in the negative control of cell proliferation primarily by upregulating the expression of p21, a potent inhibitor of cyclin-dependent kinase. In order to examine the effect of decorin on myofibroblast cell growth, we rendered a human lung myofibroblast cell line, MRC-5, quiescent by either cell-cell contact or serum starvation, and examined the relationship between decorin and alpha-SMA expression in these cells. The expression of decorin in cells made quiescent by serum starvation was lower than that in cells made quiescent by cell-cell contact. In contrast, the expression of alpha-SMA in cells made quiescent by cell-cell contact was lower than that in cells made quiescent by serum starvation. Furthermore, forced expression of decorin was accompanied by a suppression of alpha-SMA expression, whereas knocking down of decorin expression by RNA interference increased the expression of alpha-SMA.