['Hyperhomocysteinemia (HHcy) accelerates atherosclerosis and increases inflammatory monocytes (MC) in peripheral tissues. However, its causative role in atherosclerosis is not well established and its effect on vascular inflammation has not been studied. The underlying mechanism is unknown.', 'This study examined the causative role of HHcy in atherogenesis and its effect on inflammatory MC differentiation.', 'We generated a novel HHcy and hyperlipidemia mouse model, in which cystathionine β-synthase (CBS) and low-density lipoprotein receptor (LDLr) genes were deficient (Ldlr(-/-) Cbs(-/+)). Severe HHcy (plasma homocysteine (Hcy)=275 μmol/L) was induced by a high methionine diet containing sufficient basal levels of B vitamins. Plasma Hcy levels were lowered to 46 μmol/L from 244 μmol/L by vitamin supplementation, which elevated plasma folate levels. Bone marrow (BM)-derived cells were traced by the transplantation of BM cells from enhanced green fluorescent protein (EGFP) transgenic mice after sublethal irradiation of the recipient. HHcy accelerated atherosclerosis and promoted Ly6C(high) inflammatory MC differentiation of both BM and tissue origins in the aortas and peripheral tissues. It also elevated plasma levels of TNF-α, IL-6, and MCP-1; increased vessel wall MC accumulation; and increased macrophage maturation. Hcy-lowering therapy reversed HHcy-induced lesion formation, plasma cytokine increase, and blood and vessel inflammatory MC (Ly6C(high+middle)) accumulation. Plasma Hcy levels were positively correlated with plasma levels of proinflammatory cytokines. In primary mouse splenocytes, L-Hcy promoted rIFNγ-induced inflammatory MC differentiation, as well as increased TNF-α, IL-6, and superoxide anion production in inflammatory MC subsets. Antioxidants and folic acid reversed L-Hcy-induced inflammatory MC differentiation and oxidative stress in inflammatory MC subsets.'] Does severe hyperhomocysteinemia promote bone marrow-derived and resident inflammatory monocyte differentiation and atherosclerosis in LDLr/CBS-deficient mice?