One case of the so‐called „Stewart‐Treves syndrome” (STS), appearing on a lymphoedomatous arm complicating radical mastectomy for brest cancer, was characterized electronmicroscopically and immunohistologically, in order to elucidate its disputed (epithelial vs endothelial) histogenesis. Epithelial and endothelial differentiation markers used comprised: antibodies against keratin, vimentin, factor VIII‐related antigen (F VIII‐RA), HLA‐DR antigens and the lectinUlex europeausagglutinin 1 (UEA I). At the ultrastructural level, neoplastic cells were found to contain typical Weibel‐Palade bodies, whereas by immunohistological techniques they proved to be keratin −negative/vimentin+, F VIII‐RA +, UEAI+, HLA‐DR+. These results rule out a possible epithelial differentiation and strongly favour an en