OBJECTIVE: To determine whether MCP-1 (monocyte chemoattractant protein 1) levels in human atherosclerotic plaques associate with plaque vulnerability features. APPROACH AND RESULTS: We measured MCP-1 levels in human atherosclerotic plaque samples from 1199 patients in the AtheroEXPRESS Biobank who underwent endarterectomy for treatment of carotid stenosis. We explored associations with histopathologic and molecular features of plaque vulnerability, clinical plaque manifestations, and vascular events up to 3 years after endarterectomy. Following adjustments for age, sex, and vascular risk factors, MCP-1 plaque levels were associated with histopathologic markers of plaque vulnerability (large lipid core, low collagen content, high macrophage burden, low smooth muscle cell burden, intraplaque hemorrhage) and with a composite vulnerability index (range 0-5, β per SD increment in MCP-1, 0.42 [95% CI, 0.30-0.53], P=5.4×10−13). We further found significant associations with higher plaque levels of other chemokines and proinflammatory molecules and markers of neovascularization and matrix turnover. When exploring clinical plaque instability, MCP-1 plaque levels were higher among individuals with symptomatic plaques as compared with those with asymptomatic plaques (odds ratio per SD increment in MCP-1, 1.36 [95% CI, 1.09-1.69]). MCP-1 levels were further associated with a higher risk of periprocedural major adverse vascular events and strokes occurring in the first 30 days after plaque removal. CONCLUSIONS: Higher MCP-1 plaque levels are associated with histopathologic, molecular, and clinical hallmarks of plaque vulnerability in individuals undergoing carotid endarterectomy. Our findings highlight a role of MCP-1 in clinical plaque instability in humans and complement previous epidemiological, genetic, and experimental studies supporting the translational perspective of targeting MCP-1 signaling in atherosclerosis.