Background: In this study, we identified proteins that were expressed differentially in human atherosclerotic plaques. Among the differentially expressed proteins, we were interested in heat shock protein 27 (Hsp27), as there have been few studies about Hsp27 in patients with coronary artery disease. Methods and Results: We performed two-dimensional electrophoretic analysis of proteins extracted from human carotid endarterectomy specimens (n=10). Comparison of the proteome of the plaque core area with that of a nearby normal looking area in the same vessel specimen revealed that Hsp27 is the most differentially expressed protein between the two regions. Immunohistochemical analysis of carotid artery endarterectomy specimens showed co-localized expression of Hsp27 with smooth muscle cell marker and its phosphorylated form at the foam cell rich and ApoB-100 deposited region. We then measured the serum levels of Hsp27 in 27 patients with acute coronary syndrome (ACS) and compared them with the levels in 17 patients with chronic stable angina (CSA) and 29 sex and age matched normal controls. The mean plasma level of Hsp27 was significantly higher in ACS patients (106.1 +(-) 74.1 ng/mL) than in CSA patients (61.3 +(-) 47.4 ng/mL, p < 0.05) and normal control subjects (45.8 +(-) 29.5 ng/mL, p < 0.05). The level of Hsp27 demonstrated a significant correlation with the level of C-reactive protein, total cholesterol, Hsp70, and CD40L respectively. Conclusion: Hsp27 is suggested to be a potential marker of acute coronary syndrome, and may be involved in the active inflammation of the plaque.