Among vascular cell adhesion molecules, platelet-endothelial cell adhesion molecule (PECAM-1/CD31) has the distinctive feature of being expressed on several of the major cell types associated with the vascular compartment. This makes it uniquely positioned to mediate multiple and important cell-cell interactions involving platelets, leukocytes and endothelial cells. Thus, PECAM-1 may represent a potential target for new therapeutic agents directed at a variety of pathological states.