AbstractThe formation of misaligned scar tissue by a variety of cell types expressing multiple axon growth inhibitory proteoglycans presents a physical and molecular barrier to axon regeneration after adult spinal cord injuries. Decorin is a small, leucine‐rich proteoglycan that has previously been shown to reduce astrogliosis and basal lamina formation in acute cerebral cortex stab injuries. We have therefore tested whether mini pump infusion of hr‐decorin into acute stab injuries of the adult rat spinal cord can not only inhibit formation of an astroglial limitans but also deposition of the axon growth inhibitory proteoglycans neurocan, NG2, phosphacan and brevican. Combined immunohistochemical and quantitative Western blot analysis revealed major reductions in levels of core protein expression (>80% for 130‐kDa neurocan, 145/80‐kDa brevican, 300‐kDa phosphacan) and immunoreactivity for all four chondroitin sulfate proteoglycans (CSPGs) within decorin‐treated injuries compared with untreated controls. Astrogliosis within lesion margins and the accumulation of OX42+ macrophages/microglia within lesion centres were also significantly reduced. These decorin‐induced changes in scar formation combined to promote the striking ability of axons from microtransplanted adult sensory neurons to enter, grow within and exit decorin‐infused spinal cord injuries, in sharp contrast to the complete failure of axons to cross untreated, CSPG‐rich lesions. Decorin pretreatment of meningial fibroblasts in vitro also resulted in a three‐fold increase in neurite outgrowth from co‐cultured adult sensory neurons and suppression of NG2 immunoreactivity. The ability of decorin to promote axon growth across acute spinal cord injuries via a coordinated suppression of inflammation, CSPG expression and astroglial scar formation make decorin treatment a promising component of future spinal cord regeneration strategies.