CD31, a transmembrane protein expressed on endothelial and hematopoietic cells, plays important roles in leukocyte trafficking, mechanotransduction, angiogenesis, vascular permeability, and regulation of cellular responsiveness. CD31 immunoreactivity is employed as a sensitive and specific endothelial marker in diagnostic pathology. In this study, CD31 expression in myocardial tissues from deceased patients with ischemic heart disease and a mouse model of acute myocardial infarction were examined by immunohistochemical staining. We examined 24 neutral formalin-fixed, paraffin-embedded myocardial tissue samples obtained within 48 h postmortem from the autopsies of patients who were diagnosed with ischemic heart disease. CD31 expression was observed in vascular endothelial and endocardial cells. In necrotic myocardium, diffusion of CD31 antigen was observed. Elevated CD31 expression was observed around myocardial cells undergoing remodeling, suggesting that endothelial proliferation occurred at these sites. In contrast, fibrotic myocardial foci did not show upregulated CD31 expression. The same CD31 expression characteristics as those observed in the human samples were observed in the mouse model. CD31 immunostaining as an endothelial and microvasculature marker may be a useful complement to conventional staining techniques currently used in the diagnosis of ischemic heart disease, and may allow the timing and process of myocardial remodeling to be analyzed in detail.