Acute coronary syndromes (ACS) are characterized by multiple unstable coronary plaques and elevated circulating levels of inflammatory biomarkers. The endothelium of internal mammary arteries (IMA), atherosclerosis resistant, is exposed to pro-inflammatory stimuli as vessels developing atherosclerosis. Our study investigated the IMA endothelial expression of inflammatory molecules in patients with ACS or chronic stable angina (CSA). IMA demonstrated normal morphology, intact endothelial lining and strong immunoreactivity for Glut-1. E-selectin expression was observed more frequently on IMA of ACS than of CSA patients (ACS 61% vs. CSA 14%, p=0.01). High fluorescence for MHC was significantly more frequent on luminal endothelium (ACS 66.7% vs. CSA 17.6%, p=0.001 for class I; ACS 66.7% vs. CSA 6.2%, p=0.0003 for class II-DR) and on vasa vasorum (ACS 92,9% vs. CSA 33.3%, 7.7%, p=0.0007 and p<0.0001 for class I and class II-DR respectively) of ACS than of CSA patients. ICAM-1, VCAM-1, TLR-4, Tissue Factor, Interleukin-6, iNOS, and TNF expression was not significantly different in ACS and in CSA. Circulating C-reactive protein (ACS 4.8 (2.6-7.3) mg/L vs. CSA 1.8 (0.6-3.5) mg/L, p=0.01), Interleukin-6 (ACS 4.0 (2.6-5.5) pg/mL vs. CSA 1.7 (1.4-4.0) pg/mL, p=0.02) were higher in ACS than in CSA, without correlation with IMA inflammation. The higher E-selectin, MHC class I and class II-DR on endothelium and vasa vasorum of IMA from ACS patients suggests a mild, endothelial inflammatory activation in ACS, which can be unrelated to the presence of atherosclerotic coronary lesions. These findings indicated IMA as active vessels in coronary syndromes.