Monocyte chemoattractant protein-1 (MCP-1) is a member of chemokines with chemoattractant activity for monocytes, T cells, mast cells, and basophils. Precursor mRNA or protein was detected at high levels in the lesions of several diseases, such as pulmonary fibrosis, rheumatoid arthritis, atherosclerosis, and some types of tumors. The regulation of MCP-1 production and the role of this chemokine in pathophysiologic states, however, remain largely unknown. In this study, using an enzyme-linked immunosorbent assay (ELISA), we measured the circulating MCP-1 levels in 405 healthy Japanese subjects of various ages, eliciting a profound age-dependent MCP-1 increase. Multivariate regression analysis revealed that significant predictors of MCP-1 value for males were age (p = 0.033) and serum triglyceride (p = 0.039). For females, age was also a significant predictor (p = 0.00002). One possible explanation is that the plasma MCP-1 concentration might reflect the existence of atherosclerosis, although the plasma MCP-1 concentration from patients with coronary artery disease or cerebrovascular accidents appears not to differ from age-matched, disease-free controls. This is the first report linking an increase in a particular chemokine level with aging.