Blastic plasmacytoid dendritic cell neoplasm (BPDCN) is a rare malignant neoplasm with cutaneous manifestations and a rapidly progressive clinical course. The diagnosis relies on characteristic clinicopathologic and immunopathologic features. However, the overlap of immunophenotypic features with other cancers, as well as newly discovered interpersonal and intrapersonal phenotypic variations, renders the identification of BPDCN challenging. A greater understanding of the proteins expressed by BPDCN might facilitate its recognition and provide insights into its clinical behavior.In 7 of 9 patients at 4 tertiary care institutions, immunohistochemical analysis demonstrated strong CD31/PECAM-1 (platelet endothelial cell adhesion molecule 1) expression by neoplastic cells. Combined with similar findings observed in 1 former patient, 8 of 10 cases of BPDCN were CD31/PECAM-1 positive.Expression of CD31/PECAM-1 by BPDCN adds new information about the antigenic profile of this unusual neoplasm. CD31/PECAM-1 influences multiple cell functions including adhesion, apoptosis, coagulation, host response, and protein synthesis that might affect clinical features of BPDCN such as hemorrhage, aggressive tumor growth, and resistance to therapy. Therefore, the potential role of this molecule in the tumor formation and progression of BPDCN warrants additional exploration.