Macrophages play a major role in the development of vascular lesions in atherogenesis. The cells express FcgammaRIIIa (CD16) identical to that in NK cells, but with a cell type-specific glycosylation, and these soluble forms (sFcgammaRIIIa) are present in plasma. We measured sFcgammaRIIIa(Mphi) derived from macrophages in plasma from subjects undergoing an annual medical checkup. The levels of sFcgammaRIIIa(Mphi) increased with age, and correlated positively with body mass index, blood pressure, LDL cholesterol to HDL cholesterol ratio, triglycerides, hemoglobin A1c, and creatinine, but negatively with HDL-cholesterol levels. The sFcgammaRIIIa(Mphi) levels were related to the number of risk factors for atherosclerosis: such as aging, current smoking, diabetes, hypertension, hyper-LDL-cholesterolemia, hypo-HDL-cholesterolemia, and family history of atherosclerotic diseases. In addition, the sFcgammaRIIIa(Mphi) levels were correlated with carotid maximum intima-media thickness (IMT). These findings indicate the macrophages are activated during the incipient stage of atherosclerosis, and suggest sFcgammaRIIIa(Mphi) may be used as a predictive marker for atherosclerosis.