Background: Interleukin (IL)-β plays a central role in inflammation and atherosclerosis, but levels of IL-1β, its natural antagonist, IL-1Ra, and their balance in human atherosclerotic lesions, are unknown. Knowledge of protein levels in atherosclerosis and the influence of a functional IL-1Ra polymorphism would increase the understanding of atherosclerosis pathogenesis. Methods and Results: Fresh and endotoxin-stimulated explanted human atherosclerotic and normal arteries were analyzed for IL-1β, IL-1Ra and IL-1 receptor 1 (IL-1R1) using TaqMan PCR and enzyme-linked immunosorbent assay. Two hundred forty-three survivors of a first myocardial infarction were genotyped for a polymorphism in IL-1Ra and their coronary atherosclerosis analyzed by using coronary angiography. Levels of IL-1β, IL-1Ra and IL-1R1 mRNA were significantly increased in atherosclerotic arteries compared with normal arteries. Endotoxin stimulation increased IL-1β levels more than IL-1Ra levels (ie, promoted a pro-inflammatory state). A polymorphism in IL-1Ra known to increase levels of IL-1Ra was associated with decreased mean coronary artery plaque area. Conclusions: Activation of innate immunity changed the balance between IL-1β and IL-1Ra in atherosclerotic arteries towards a more pro-inflammatory state. In line with this, the presence of an IL-1Ra intron 2 polymorphism known to increase IL-1Ra levels, and possibly the IL-1Ra:IL-1β ratio, was associated with reduced coronary atherosclerosis. (Circ J 2009; 73: 1531 - 1536)