Aim To compare the expression of matrix metalloproteinase-2(MMP-2)and MMP-9 in atherosclerotic plaques of diabetic and non-diabetic patients and to investigate the potential role of MMP-2 and MMP-9 expression in unstabilizing plaques of diabetic patients. Methods Specimens of arteries were obtained from 23 diabetic patients and from 17 autopsy cases with acute coronary syndromes. 126 tissue blocks of late-stage lesions were classified into diabetic group (n=74)and non-diabetic group (n=52). Forty blocks were selected randomly from each group and serial sections were stained immunohistochemically with anti-human MMP-2 and MMP-9 monoclonal antibody. Microscope Image Analyzer was used for relatively quantitative analysis. Results In the diabetic arteries, intense staining was shown around the plaque core, especially in the plaque shoulder and thinned fibrous cap. MMP-2 integrated optical density(IOD) of diabetic group (69 014±14 459) was strikingly higher than that of the non-diabetic group (57 004±16 171). Threshold area of MMP-2 was significantly elevated (p<0.05) in samples from diabetic group (13.0%±2.7%) compared with samples from non-diabetic group with atherosclerosis (11.1%± 3.3% ). IOD of MMP-9 was also significantly higher (p<0.01) in diabetic group (102 485±20 431) than in the non-diabetic group (75 280±13 106). Conclusion The expression of MMP-2 and MMP-9 in atherosclerotic plaques of diabetic group is significantly higher than that of non-diabetic group. MMP-2 and MMP-9 accumulation, which play an important role in unstabilizing plaques, may promote atherosclerotic plaque disruption in diabetic patients.