By activating immune cells or a direct action on the vascular wall, leptin may affect the initiation and progression of atherosclerosis. We investigated whether plasma leptin concentration is associated with coronary artery disease, with particular focus on the relationship between plasma leptin and the development of an acute coronary syndrome. Plasma leptin, interleukin-6 and high-sensitivity C-reactive protein were measured in 34 patients with acute coronary syndrome and 21 with stable angina. Their results were compared with those of 21 normal controls. Plasma leptin levels were significantly higher in the acute coronary syndrome group (13.36 +/- 5.02 ng.mL(-1)) compared to the stable angina group (8.97 +/- 4.06 ng.mL(-1)) or normal controls (5.14 +/- 2.75 ng.mL(-1)). Interleukin-6 and high-sensitivity C-reactive protein were also higher in the acute coronary syndrome group, and leptin correlated positively with interleukin-6 and high-sensitivity C-reactive protein. These findings suggest that plasma leptin levels may be a useful marker of systemic inflammation, and measurement of plasma leptin may be helpful in assessing the risk of developing coronary heart disease.