Background Tenascin-C (TNC) is an extracellular matrix glycoprotein that increases after inflammation and injury. In cultured cells TNC has been reported to markedly induce the expression of matrix metalloproteinase-9, which stimulates collagen degradation in the fibrous cap of human atherosclerotic plaque. Methods and Results Immunohistochemical techniques were used to analyze the expression of TNC protein in 51 coronary atherectomy specimens obtained from patients with stable angina pectoris (SAP, n=23) or acute coronary syndromes (ACS) (n=28; unstable angina pectoris, n=20, acute myocardial infarction, n=8). Immunostaining for α-smooth muscle actin, CD68, CD45, and CD31 was also performed in serial sections to identify the cell types that express TNC protein. The %TNC + area (percentage of the area of immunostaining for TNC protein in the total surface area of the plaque) was larger in coronary samples with the plaque characteristics of thrombus, angiogenesis, intraplaque hemorrhage, and macrophage (CD68⁺), and lymphocyte (CD45 ⁺) clusters than in coronary samples without them (52±3.4 vs 39±4.8, p<0.05; 57±3.7 vs 36±3.7, p<0.01; 51±3.6 vs 39±4.8, p<0.05; 53±3.4 vs 33±4.5, p<0.01; 56±4.1 vs 37±3.6, p<0.01, respectively). The presence of other components, such as dense fibrous tissue, neointimal hyperplasia, atheromatous gruel and calcification, was not significantly correlated with the %TNC + area. The %TNC + area was larger in coronary samples from patients with ACS than in samples from patients with SAP (56±3.2% vs 34±4.3%, p<0.01). Conclusions The results suggest that TNC may have specific functions in coronary plaque formation and may be involved in the pathogenesis of coronary lesions in ACS. (Circ J 2004; 68: 198 - 203)