Irisin, a novel myokine, was proposed to be able to regulate glucose homeostasis and obesity in mice. Whether irisin levels are associated with cardio-metabolic variables, insulin sensitivity, and vascular atherosclerosis in humans remain unsettled. To determine the associations between circulating irisin levels, cardio-metabolic variables, insulin sensitivity, and common carotid intima-media thickness (IMT), an indicator of vascular atherosclerosis, a cross-sectional evaluation of circulating irisin levels and cardio-metabolic variables in 192 White adults was conducted. Insulin sensitivity and insulin clearance were assessed by euglycemic-hyperinsulinemic clamp. Common carotid IMT was measured by ultrasound. After adjusting for age and gender, irisin levels were positively correlated with body fat mass (r = 0.12, P < 0.05), fasting (r = 0.17, P < 0.01), 2 h post-load insulin (r = 0.15, P < 0.02) levels, and IMT (r = 0.29, P < 0.0001) and were negatively correlated with insulin-stimulated glucose disposal (r = -0.18, P = 0.007), Matsuda index (r = -0.13, P < 0.04), disposition index (r = -0.278, P < 0.0001), and insulin clearance (r = -0.26, P < 0.0001). After adjusting for age, gender, and BMI, individuals in the highest tertile of irisin levels exhibited higher body fat mass (P < 0.01), fasting (P < 0.05), 2 h post-load (P < 0.01) insulin levels, carotid IMT (P < 0.001), lower insulin-stimulated glucose disposal (P < 0.001), Matsuda index (P < 0.01), disposition index (P < 0.01), and insulin clearance (P < 0.001) as compared with subjects in the lowest tertile of circulating irisin levels. Irisin is inversely associated with insulin sensitivity and positively associated with carotid IMT in humans, suggesting either increased release by adipose/muscle tissue in response to deterioration of insulin sensitivity or a compensatory increase in irisin to overcome an underlying irisin resistance.