Mast cells contribute importantly to allergic and innate immune responses by releasing various preformed and newly synthesized mediators^,. Previous studies have shown mast cell accumulation in human atherosclerotic lesions. This report establishes the direct participation of mast cells in atherogenesis in low-density lipoprotein receptor–deficient (Ldlr^−/−) mice. Atheromata from compound mutant Ldlr^−/−Kit^W-sh/W-sh mice showed decreased lesion size, lipid deposition, T-cell and macrophage numbers, cell proliferation and apoptosis, but increased collagen content and fibrous cap development. In vivo, adoptive transfer of syngeneic wild-type or tumor necrosis factor (TNF)-α-deficient mast cells restored atherogenesis to Ldlr^−/−Kit^W-sh/W-sh mice. Notably, neither interleukin (IL)-6- nor interferon (IFN)-γ-deficient mast cells did so, indicating that the inhibition of atherogenesis in Ldlr^−/−Kit^W-sh/W-sh mice resulted from the absence of mast cells and mast cell–derived IL-6 and IFN-γ. Compared with wild-type or TNF-α-deficient mast cells, those lacking IL-6 or IFN-γ did not induce expression of proatherogenic cysteine proteinase cathepsins from vascular cells in vitro or affect cathepsin and matrix metalloproteinase activities in atherosclerotic lesions, implying that mast cell–derived IL-6 and IFN-γ promote atherogenesis by augmenting the expression of matrix-degrading proteases. These observations establish direct participation of mast cells and mast cell–derived IL-6 and IFN-γ in mouse atherogenesis and provide new mechanistic insight into the pathogenesis of this common disease.