AbstractGanoderma lucidum is used in traditional Chinese medicine to prevent or treat a variety of diseases, including cardiovascular disorders. We previously demonstrated that a glucan‐containing extract of Reishi polysaccharides (EORP) has the potent anti‐inflammatory action of reducing ICAM‐1 expression in lipopolysaccharide (LPS)‐treated human aortic smooth muscle cells (HASMCs) and LPS‐treated mice. In the present study, we examined whether EORP inhibited platelet‐derived growth factor‐BB (PDGF)‐stimulated HASMC proliferation and the mechanism involved. EORP dose‐dependently reduced cell numbers and DNA synthesis of PDGF‐treated HASMCs in vitro. EORP also arrested cell cycle progression in the G0/G1 phase, and this was associated with decreased expression of cyclin D1, cyclin E, CDK2, CDK4, and p21Cip1 and upregulation of the cyclin‐dependent kinase inhibitor p27Kip1. The anti‐proliferative effect of EORP was partly mediated by downregulation of PDGF‐induced JNK phosphorylation. In in vivo studies, the femoral artery of C57BL/6 mice was endothelial‐denuded and the mice were fed a diet containing 100 mg/kg/day of EORP. On day 14, both cell proliferation (proliferating cell nuclear antigen‐positive cells) in the neointima and the neointima/media area ratio (0.67 ± 0.03 vs. 1.46 ± 0.30) were significantly reduced. Our data show that EORP interferes with the mitogenic activation of JNK, preventing entry of HASMCs into the cell cycle in vitro and reducing cell proliferation in the neointima and decreasing the neointimal area in vivo. Thus, EORP may represent a safe and effective novel approach to the prevention and treatment of vascular proliferative diseases. J. Cell. Physiol. 227: 3063–3071, 2012. © 2011 Wiley Periodicals, Inc.