Platelet-endothelial cell adhesion molecule-1 protein (PECAM-1/CD31) is expressed in numerous physiological and pathological processes characterized by an increase of vascular permeability, and in normal and tumour tissues. CD31, member of the immunoglobulin super-family that mediates cell-to-cell adhesion, is a transmembrane glycoprotein, 130-140 kDa, also know as platelet-endothelium cell adhesion molecule (PECAM-1). CD31 is a ligand for CD38 and plays a role in thrombosis and angiogenesis. CD31 is strongly expressed in endothelial cells and weakly expressed in megakaryocytes, platelets, occasional plasma cells, lymphocytes (marginal zone B-cells, peripheral T-cells) and neutrophils. The present study evaluates the angiogenetic processes which are accompanied by an expansion of cystic radicular and keratocystic lesions of the jaw bone. Twelve subjects with maxillary cysts (8 males and 4 females) with an average age of 43 years were selected by the Chieti University Oral Surgery Department. The surgical samples taken were subjected to histological and immunohistochemical analysis. The histological evaluation confirmed the diagnosis of radicular cystisis and keratocystisis. The immunohistochemical analyses were positive for CD31 protein in all the lesions analysed, even though they had different intensities. Using a semiquantive analysis it was possible to highlight, in the radicular cyst samples, an intense expression of the vascular component both in the inflamed area and the adjacent stroma. The lesions with cheratin content showed newly-formed, rather modest, vascularity both in the area showing slight inflammation, where the cellular component is prevalent, and in the adjacent areas showing no sign of inflammation. Therefore, in our observations, angiogenesis could take on a primary role in the development of cystic lesions of the jaw bones. The differences of CD31 expression, in all samples, would advise for a wider monitoring able to evaluate the possible use of such a protein as a diagnostic marker.