Aim To analyse the relation between heparanase (HPA) and stablilization of athersclerotic plaques in human coronary arteries; and to investigate the role of HPA in creating vulenerable sites in atherosclerotic plaques. Methods Samples of coronary arteries were obtained at autopsies of 36 patients with acute coronary syndrome. Eighty randomly selected blocks were studied by immunohistochemistry using antibody against HPA. Computer-aid planimetor was used for quantitative analysis. Results In unstable plaques, percentage of immunoreactive areas for HPA was significantly higer than that in stable plaques. Conclusions HPA in coronary atherosclerotic plaque is an important factor in determining stablilization of the plaques.