PECAM-1 (platelet endothelial cell adhesion molecule-1) also called CD31 and EndoCAM is a newly characterized adhesion molecule that belongs to the immunoglobulin superfamily . PECAM-1 is a transmembrane glycoprotein with a molecular weight of approximately 130 kDa, depending on the degree of glycosylation. PECAM-1 is constitutively expressed on all vascular cells and has provided a useful immunohistochemical marker of blood vessels, particularly in the setting of angiogenesis. It has also been found on platelets, monocytes, neutrophils and CD8+ T cells. Bone marrow stem cells and transformed cell lines of the myeloid and megakaryocytic lineage also express PECAM-1. Interestingly, PECAM-1 was also detected on human, mouse and rat solid tumor lines . Recent studies suggest a role for PECAM-1 in the inflammatory process and leukocyte-endothelial interaction. The process of leukocyte emigration to the site of inflammation can be dissected into three successive stages: rolling, mediated by the selectins; tight adhesion mediated by ICAMs and their counter-receptors, the integrins; and transmigration of leukocytes through intercellular junctions of vascular endothelial cells which requires PECAM-1. PECAM-1 appears to be able to interact both with itself (homophilic binding) and with other “non-PECAM-1” molecules (heterophilic binding). PECAM-1 is an early and sensitive marker for tumor-induced angiogenesis. Several data have suggested that PECAM-1 may be involved in the process of angiogenesis in a developing vertebrate embryo as well as during metastases formation. Besides the membrane-bound form of PECAM-1 a soluble form of the molecule exists, which is 5–10 kDa smaller than cell-associated PECAM-1, and contains the cytoplasmic tail. This form of soluble PECAM-1 is encoded by an alternatively spliced mRNA from which the exon containing the transmembrane domain has been removed. Soluble PECAM-1 was detected in normal human plasma. Antibodies against PECAM-1 have been shown to exhibit a high degree of sensitivity and specificity for endothelial cells in normal, inflammatory and neoplastic tissue. PECAM-1 is a sensitive and specific marker for detection of melanoma-associated angiogenesis. Thus, PECAM-1 may be a useful marker for early detection of tumor metastasis. Compared to normal gut, there is a significant increase of PECAM-1-positive vessels in the mucosa of uninvolved ulcerative colitis. In gingivitis and periodontitis lesion the expression of PECAM-1 on mononuclear infiltrates increases significantly with increasing size of infiltrate.